Use ty instead of MyPy for type checking

[bittner]

Replaces the mypy static type checker by the modern, ultra-fast ty in the Tox configuration and GHA setup.

[bittner]

@copilot The static type checker ty found a few issues. Please, suggest how to fix them, e.g. by adding type annotations.

Hints

• Run ty via tox run -e types (which installs dependencies that allow ty to evaluate imports in the test code)

[Copilot]

@bittner I've opened a new pull request, #117, to work on those changes. Once the pull request is ready, I'll request review from you.