Upgrade to Python 3.8 / drop Python 3.7

[snejus]

Description

Fixes #5222.

Drop Python 3.7. pyupgrade is responsible for most of the changes in the code. I undid some of the bits it attempted to update that aren't strictly necessary:

1. Converting List/Dict/Tuple -> list/dict/tuple in modules that have from __future__ import annotations import. This should be done in a separate PR, and for all modules
2. Converting some .format( calls to f-strings. It didn't do it consistently, and it should also be done in a separate PR, I believe.

Python upgrade unblocks several other PRs, for example #5266 and #5248.

To Do

• Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• Tests. (Very much encouraged but not strictly required.)

[Serene-Arc]

Changes seem good to me. Nice that 3.8 has its own lazy property functionality in functools.