Solve all the type issues

[eyeseast]

Closes #116
Closes #112
Closes #121

Much more type checking, trying to be strict about things. Ultimately lots of changes that should amount to no actual change in functionality.

[eyeseast]

This is passing everywhere except Python 3.9, which hits end-of-life in October. The issue is typing.TypeGuard, which was added in 3.10. Three options, then:

• Drop 3.9 early
• Wait to release this until October
• Type check without using TypeGuard

[liffiton]

If you're taking votes, I'd vote for dropping 3.9 early. Folks on 3.9 can keep using the existing release.

It would be tricky if you wanted to keep supporting 3.9 with other updates, but this is a simple, stable library that doesn't need frequent updates.

[eyeseast]

I might make a 2.0 branch that I can merge this into, and work on all the other overdue updates, with a plan to release that all at once in October when 3.9 goes off to the land of unsupported software.