Merge #19504: Bump minimum python version to 3.6

97c738ff1b592270491551cc0a43472d244ffbb0 [tests] Recommend f-strings for formatting, update feature_block to use them (Anthony Towns)
8ae9d314e9af7bcce1e8bc52f0317b9d565109bf Bump minimum python version to 3.6 (Anthony Towns)

Pull request description:

  Python 3.5 has reached [end-of-life](https://devguide.python.org/#status-of-python-branches) as of September 2020, and 3.6 has some moderately nice [features](https://docs.python.org/3/whatsnew/3.6.html):

  - `f'x = {x}'` as an alternative to `'x = {}'.format(x)` format strings (cf https://github.com/bitcoin/bitcoin/pull/13718#issuecomment-406591027)
  - underscore separators for large numbers, like `1_234_567`
  - improvements to async
  - improvements to typing module

  Note that 3.6 is not available in xenial (16.04), but is available in bionic (18.04), while focal (20.04) has 3.8. CentOS 7 and 8 have 3.6.8, Debian stable has 3.7.3, and [gentoo and arch already had 3.6 and 3.7 in 2018](https://github.com/bitcoin/bitcoin/pull/14954#issuecomment-447118707).

ACKs for top commit:
  MarcoFalke:
    re-ACK 97c738ff1b

Tree-SHA512: ec7fce68845edde4d61a42de12c065fd49e5217311a6fda1323206f091a0afd50f293645dffc27d420127e4e5deb864e953f1b67eff735a0dfbbedd7899a9d60

7 files changed, 15 insertions, 15 deletions

diff --git a/.python-version b/.python-version
index c49282585a..8b7b0b52e5 100644
--- a/.python-version
+++ b/.python-version
@@ -1 +1 @@
-3.5.6
+3.6.12
diff --git a/.travis.yml b/.travis.yml
index 656eed9871..bb5e0a854c 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -57,7 +57,7 @@ jobs:
       env:
       cache: pip
       language: python
-      python: '3.5' # Oldest supported version according to doc/dependencies.md
+      python: '3.6' # Oldest supported version according to doc/dependencies.md
       install:
         - set -o errexit; source ./ci/lint/04_install.sh
       before_script:
@@ -71,6 +71,6 @@ jobs:
         FILE_ENV="./ci/test/00_setup_env_i686_centos.sh"
 
     - stage: test
-      name: 'x86_64 Linux  [GOAL: install]  [xenial]  [no wallet]'
+      name: 'x86_64 Linux  [GOAL: install]  [bionic]  [no wallet]'
       env: >-
         FILE_ENV="./ci/test/00_setup_env_native_nowallet.sh"
diff --git a/ci/test/00_setup_env_native_nowallet.sh b/ci/test/00_setup_env_native_nowallet.sh
index 0a09bfe230..0fe7d35435 100644
--- a/ci/test/00_setup_env_native_nowallet.sh
+++ b/ci/test/00_setup_env_native_nowallet.sh
@@ -7,8 +7,8 @@
 export LC_ALL=C.UTF-8
 
 export CONTAINER_NAME=ci_native_nowallet
-export DOCKER_NAME_TAG=ubuntu:16.04  # Use xenial to have one config run the tests in python3.5, see doc/dependencies.md
-export PACKAGES="python3-zmq clang-3.8 llvm-3.8"  # Use clang-3.8 to test C++11 compatibility, see doc/dependencies.md
+export DOCKER_NAME_TAG=ubuntu:18.04  # Use bionic to have one config run the tests in python3.6, see doc/dependencies.md (3.6 is not available in xenial/16.04)
+export PACKAGES="python3-zmq clang-3.9 llvm-3.9"  # Use clang-3.9 to test C++11 compatibility, see doc/dependencies.md (3.8 is not available in bionic)
 export DEP_OPTS="NO_WALLET=1"
 export GOAL="install"
-export BITCOIN_CONFIG="--enable-glibc-back-compat --enable-reduce-exports CC=clang-3.8 CXX=clang++-3.8 --with-boost-process"
+export BITCOIN_CONFIG="--enable-glibc-back-compat --enable-reduce-exports CC=clang-3.9 CXX=clang++-3.9 --with-boost-process"
diff --git a/configure.ac b/configure.ac
index b63c3514c7..01041ceab1 100644
--- a/configure.ac
+++ b/configure.ac
@@ -107,8 +107,8 @@ AC_PATH_TOOL(STRIP, strip)
 AC_PATH_TOOL(GCOV, gcov)
 AC_PATH_TOOL(LLVM_COV, llvm-cov)
 AC_PATH_PROG(LCOV, lcov)
-dnl Python 3.5 is specified in .python-version and should be used if available, see doc/dependencies.md
-AC_PATH_PROGS([PYTHON], [python3.5 python3.6 python3.7 python3.8 python3 python])
+dnl Python 3.6 is specified in .python-version and should be used if available, see doc/dependencies.md
+AC_PATH_PROGS([PYTHON], [python3.6 python3.7 python3.8 python3.9 python3 python])
 AC_PATH_PROG(GENHTML, genhtml)
 AC_PATH_PROG([GIT], [git])
 AC_PATH_PROG(CCACHE,ccache)
diff --git a/doc/dependencies.md b/doc/dependencies.md
index d1bf4b3a87..08fbf6145d 100644
--- a/doc/dependencies.md
+++ b/doc/dependencies.md
@@ -18,7 +18,7 @@ These are the dependencies currently used by Bitcoin Core. You can find instruct
 | librsvg | |  |  |  |  |
 | MiniUPnPc | [2.0.20180203](https://miniupnp.tuxfamily.org/files) |  | No |  |  |
 | PCRE |  |  |  |  | [Yes](https://github.com/bitcoin/bitcoin/blob/master/depends/packages/qt.mk) |
-| Python (tests) |  | [3.5](https://www.python.org/downloads) |  |  |  |
+| Python (tests) |  | [3.6](https://www.python.org/downloads) |  |  |  |
 | qrencode | [3.4.4](https://fukuchi.org/works/qrencode) |  | No |  |  |
 | Qt | [5.9.8](https://download.qt.io/official_releases/qt/) | [5.5.1](https://github.com/bitcoin/bitcoin/issues/13478) | No |  |  |
 | SQLite | [3.32.1](https://sqlite.org/download.html) | [3.7.17](https://github.com/bitcoin/bitcoin/pull/19077) |  |  |  |
diff --git a/test/functional/README.md b/test/functional/README.md
index 82b30fed51..2764acbf18 100644
--- a/test/functional/README.md
+++ b/test/functional/README.md
@@ -34,7 +34,7 @@ don't have test cases for.
 - When subclassing the BitcoinTestFramework, place overrides for the
   `set_test_params()`, `add_options()` and `setup_xxxx()` methods at the top of
   the subclass, then locally-defined helper methods, then the `run_test()` method.
-- Use `'{}'.format(x)` for string formatting, not `'%s' % x`.
+- Use `f'{x}'` for string formatting in preference to `'{}'.format(x)` or `'%s' % x`.
 
 #### Naming guidelines
 
diff --git a/test/functional/feature_block.py b/test/functional/feature_block.py
index 3cf7f3890c..158efb52c9 100755
--- a/test/functional/feature_block.py
+++ b/test/functional/feature_block.py
@@ -119,7 +119,7 @@ class FullBlockTest(BitcoinTestFramework):
         # Allow the block to mature
         blocks = []
         for i in range(NUM_BUFFER_BLOCKS_TO_GENERATE):
-            blocks.append(self.next_block("maturitybuffer.{}".format(i)))
+            blocks.append(self.next_block(f"maturitybuffer.{i}"))
             self.save_spendable_output()
         self.send_blocks(blocks)
 
@@ -151,8 +151,8 @@ class FullBlockTest(BitcoinTestFramework):
             if template.valid_in_block:
                 continue
 
-            self.log.info("Reject block with invalid tx: %s", TxTemplate.__name__)
-            blockname = "for_invalid.%s" % TxTemplate.__name__
+            self.log.info(f"Reject block with invalid tx: {TxTemplate.__name__}")
+            blockname = f"for_invalid.{TxTemplate.__name__}"
             badblock = self.next_block(blockname)
             badtx = template.get_tx()
             if TxTemplate != invalid_txs.InputMissing:
@@ -1355,12 +1355,12 @@ class FullBlockTest(BitcoinTestFramework):
 
     # save the current tip so it can be spent by a later block
     def save_spendable_output(self):
-        self.log.debug("saving spendable output %s" % self.tip.vtx[0])
+        self.log.debug(f"saving spendable output {self.tip.vtx[0]}")
         self.spendable_outputs.append(self.tip)
 
     # get an output that we previously marked as spendable
     def get_spendable_output(self):
-        self.log.debug("getting spendable output %s" % self.spendable_outputs[0].vtx[0])
+        self.log.debug(f"getting spendable output {self.spendable_outputs[0].vtx[0]}")
         return self.spendable_outputs.pop(0).vtx[0]
 
     # move the tip back to a previous block