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.

testing/code/test_excinfo.py

@@ -589,7 +589,8 @@ raise ValueError()
 
     def test_repr_local_with_exception_in_class_property(self):
         class ExceptionWithBrokenClass(Exception):
-            @property
+            # Type ignored because it's bypassed intentionally.
+            @property  # type: ignore
             def __class__(self):
                 raise TypeError("boom!")