Drop support for Python 3.7

[pradyunsg]

Python 3.7 goes EOL in 2 months and 3 weeks (https://endoflife.date/python). Filing an issue to track dropping support.

[pradyunsg]

For posterity, https://github.com/pypa/pyproject-hooks/commit/69ed8b1dac92e33d6a2d18c1b3a3cfad57020d90 is a commit with most of the work toward this. The missing bits are dropping the string literal for "SubprocessRunner" and directly documenting the protocol class as well.