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

2 files changed, 6 insertions, 6 deletions

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