Merge #20954: test: Declare `nodes` type in test_framework.py.

5353b0c64d32e44fc411464e080d4b00fae7124e Change type definitions for "chain" and "setup_clean_chain" from type comments to Python 3.6+ types. Additionally, set type for "nodes". (Kiminuo)

Pull request description:

  ### Motivation

  When I wanted to understand better https://github.com/bitcoin/bitcoin/pull/19145/files#diff-4bebbd3b112dc222ea7e75ef051838ceffcee63b9e9234a98a4cc7251d34451b test, I noticed that navigation in PyCharm/VS Code did not work for `nodes` variable. I think this is frustrating, especially for newcomers.

  ### Summary

  * This PR modifies Python 3.5 [type comments](https://mypy.readthedocs.io/en/stable/cheat_sheet_py3.html#variables) to Python 3.6+ types and adds a proper type for `nodes` [instance attribute](https://mypy.readthedocs.io/en/stable/class_basics.html#instance-and-class-attributes).
  * This PR does not change behavior.
  * This PR is intentionally very small, if the concept is accepted, a follow-up PRs can be more ambitious.

  ### End result

  1. Open `test/functional/feature_abortnode.py`
  2. Move your caret to: `self.nodes[0].generate[caret here](3)`
  3. Use "Go to definition" [F12] should work now.

  I have tested this on PyCharm (Windows, Ubuntu) and VS Code (Windows, Ubuntu).

  Note: Some `TestNode` methods (e.g. `self.nodes[0].getblock(...)` ) use `__call__` mechanism and navigation does not work for them even with this PR.

ACKs for top commit:
  laanwj:
    ACK 5353b0c64d32e44fc411464e080d4b00fae7124e
  theStack:
    ACK 5353b0c64d32e44fc411464e080d4b00fae7124e

Tree-SHA512: 821773f052ab9b2889dc357d38c59407a4af09e3b86d7134fcca7d78e5edf3a5ede9bfb37595ea97caf9ebfcbda372bcf73763b7f89b0677670f21b3e396a12b

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