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/github_helpers.py

@@ -14,8 +14,8 @@ import os
 from typing import Dict, Generator, Optional, Tuple
 
 # https://pypi.org/project/gql/
-import gql
-import gql.transport.requests
+import gql  # type: ignore
+import gql.transport.requests  # type: ignore
 
 _ENV_TOKEN = "GITHUB_ACCESS_TOKEN"
 
@@ -52,7 +52,7 @@ class Client(object):
             url="https://api.github.com/graphql",
             headers={"Authorization": "bearer %s" % parsed_args.access_token},
         )
-        self._client = gql.Client(transport=transport)  # type: ignore
+        self._client = gql.Client(transport=transport)
 
     def execute(self, query: str) -> Dict:
         """Runs a query."""