Add some typing changes required by Mypyc

[nickdrozd]

I tried compiling Coverage with Mypyc, the compiler that comes with Mypy. Mypyc is pretty stringent about what it expects, and addressing some of its objections will require some design thought. But these changes are pretty straightforward.

[nickdrozd]

https://github.com/nedbat/coveragepy/issues/1659

[nickdrozd]

@nedbat Any interest in this one? If not, I can close it.