Improve type checking

[ThunderKey]

I did some improvements regarding type checking. The main things include:

• Added mypy to verify that the types are used correctly
• Fixed issues mypy detected
  • Note: in the tests I checked the types with assert because mypy does not yet support analysing assertIsNotNone and similar functions (see assertIsNotNone(foo) in tests is not treated the same as assert foo is not None python/mypy#5088 and the linked issue Narrow types using overload information python/mypy#4063)
• Explicitly re-export functions and types from the main package (see 94b572b)
• Most importantly: added a py.typed marker to denote that this library has type hints (see PEP 561)

If you're not interested in some parts of this PR I can create a new one with only the parts you're interested in.

[JelleZijlstra]

Thanks! Ideally I'd want to typecheck using pyanalyze, but seems like that still runs into some bugs at the moment.