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, 69ed8b1 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.

[henryiii]

I'd also like the types to be available at runtime for pypa/build, Python 3.7 support is preventing this.

[notatallshaw]

This was completed in #214