Check types with mypy

[hugovk]

Helps #2625.

Run mypy to check type hints:

tox -e mypy

Some good advice here:

• https://mypy.readthedocs.io/en/stable/existing_code.html

So let's start off small with just the src directory, and exclude lots of files with warnings, currently 61, so it's running on 34 to begin with.

This adds some fairly lenient settings (happy to tweak them now and later), notably follow_imports = "silent".

When mypy encounters an import, it also checks the imported file, and its imports. They recommend using the default "normal", but lots of things import others which aren't ready yet, so let's change it when we can.

• https://mypy.readthedocs.io/en/stable/running_mypy.html#following-imports

[hugovk]

Rebased after #7624, and removed 44 files from the exclude, it now checks 76 files!