Consider changes needed/possible when Python 3.7 reaches end of life

[neiljp]

This is the next iteration of #984, and is mainly for tracking. See that issue or those prior for potentially useful tools.

3.7 is due to reach end of life on 27 Jun 2023 according to https://endoflife.date/python or more officially https://devguide.python.org/#status-of-python-branches

[neiljp]

Python 3.8 features to consider:

• Walrus syntax (unclear of benefits; was controversial?)
• Positional-only parameters
• Self-documenting f-strings (mainly for debugging?)
• TypedDict, Literal, Protocol, Final - are all in the standard library, not externally

Python 3.8 should also be generally faster at regular operations, based on the simple benchmark introduced:
https://docs.python.org/3/whatsnew/3.8.html

[neiljp]

This will also unblock (if appropriate) isort 5.12.

[neiljp]

pygments 2.18.0 (unreleased) will depend upon 3.8.

[neiljp]

pygments 2.18 is now released and supports Lean 4, which may be relevant to many users.