This basic mypy [1] configuration suppresses all type-checking errors
that it would currently report given the codebase as-is.

The idea is to re-enable those errors one by one and then fix them,
the ultimate goal being to make maintenance easier (due to editors
making more helpful suggestions and diagnosis) and uncover hidden bugs.

Besides, add a `files` directive so one can run `mypy` without
arguments.

Requires `extra/mypy` [2].

[1]: https://www.mypy-lang.org/
[2]: https://archlinux.org/packages/extra/any/mypy/