Fix mypy 1.16 errors

[DimitriPapadopoulos]

Start fixing mypy 1.16 errors.

TODO:

• [ ] Add unit tests and/or doctests in docstrings
• [ ] Add docstrings and API docs for any new/modified user-facing classes and functions
• [ ] New/modified features documented in docs/user-guide/*.rst
• [ ] Changes documented as a new file in changes/
• [x] GitHub Actions have all passed
• [x] Test coverage is 100% (Codecov passes)