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/test_pdb.py

@@ -6,7 +6,8 @@ import pytest
 from _pytest.debugging import _validate_usepdb_cls
 
 try:
-    breakpoint
+    # Type ignored for Python <= 3.6.
+    breakpoint  # type: ignore
 except NameError:
     SUPPORTS_BREAKPOINT_BUILTIN = False
 else: