Add a very lax mypy configuration, add it to tox -e linting, and
fix/ignore the few errors that come up. The idea is to get it running
before diving in too much.
This enables:
- Progressively adding type annotations and enabling more strict
options, which will improve the codebase (IMO).
- Annotating the public API in-line, and eventually exposing it to
library users who use type checkers (with a py.typed file).
Though, none of this is done yet.
Refs https://github.com/pytest-dev/pytest/issues/3342.
