Switch mypy approach to pre-commit (#1234)

I'm treating my PR to mypy bazel integration as dead in the water. Abandoning that approach, this seems to work reasonably well as a replacement. It's actually a slightly newer version of mypy too -- the bazel integration was stuck on an old version.

Note I don't think we're losing much: we shouldn't be writing too much python.

github_tools/pr_comments.py

@@ -25,6 +25,7 @@ except ImportError:
         "github_helpers",
         os.path.join(os.path.dirname(__file__), "github_helpers.py"),
     )
+    assert github_helpers_spec is not None
     github_helpers = importlib.util.module_from_spec(github_helpers_spec)
     github_helpers_spec.loader.exec_module(github_helpers)  # type: ignore