Add rudimentary mypy type checking

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.

.gitignore

@@ -35,6 +35,7 @@ env/
 .tox
 .cache
 .pytest_cache
+.mypy_cache
 .coverage
 .coverage.*
 coverage.xml