Add mypy strict typing

[jhnstrk]

This PR adds typing sufficient to pass mypy --strict checks.

The changes attempt to keep the public-facing API as close to unchanged as possible.

Internally there are some points that required design choices that are debatable:

• Use of assert in the code for type narrowing: is this "forbidden" here, or not? A few instances were added here. It may have a small performance impact, and could cause some existing code to fail.
• Some use of typing.cast. This should not have an impact, but could also be replaced with # type: ignore comments.
• Strict typing in the tests - currently the source in tests pass mypy, but I'm not sure test code needs to pass type checks.

I also added mypy to the Github actions.

With the changes, the unit tests pass, and performance (parsing throughput) seems unchanged.

[Kludex]

We need to introduce ruff format --check on the pipeline before this. I didn't notice the format was not being enforced.

I'll address the questions you asked on the description soon - it may be faster for me to just apply the answers in code.

Thanks for the PR @jhnstrk . This is really helpful 👍

[Kludex]

We need to add the py.typed file on this PR. I'll review in the next weeks.

[jhnstrk]

Thanks for the update.
I added the py.typed file, rebased on the current master (some conflicts) and fixed the formatting flagged by ruff.