Improve type hints

[marcelm]

• Run mypy on tests/
• Type-annotate the tests (add a -> None to most of the functions) so that mypy checks them
• Add overloads for open() as suggested in #94. To make mypy happy, I needed to add a _kwargs argument to open().
• Add configuration to make mypy a little bit stricter (and remove the unused type: ignore comments that this found)

CC @fjossandon

[marcelm]

Thanks! I always have to read up on how this is done (in particular overloads) because I do this so rarely.

I also added -> None to the test functions in test_open.py to get them typechecked as well. This required me to drop the deprecated file1 and file2 arguments from the overloads and from those places where they are still used in the tests. I spent way too much time on trying to make it work, but it was easier to just do it this way.

I’ll look into the coverage issue in a separate PR.