Use `mypy` to enforce type checking

[mypy](https://mypy.readthedocs.io/en/stable/) is a static type checking tool for Python. It analyses your code to check for type-related mistakes (e.g. if you try to add a `str` to an `int`), which can help catch mistakes early. However, it does mean that you have to provide [type hints](https://docs.python.org/3/library/typing.html) all over the place, which not everyone wants to do!

I'm mentioning it here because we use it as standard in the RSE team, but the choice of whether to use it or not is up to you guys @sophiemosselmans and @sanjpand!

Just let me know if you're interested.

The issue names no files, tests, or entry points. First confirm whether the project wants to adopt mypy and determine the scope of type hints needed across the Python codebase. Done means the adoption decision is made and mypy type checking is enforced consistently, but the issue does not define the configuration or validation steps.

Written by the indexing model from the issue text.