ci: re-enable Black enforcement

Summary:
Previous commits have reformatted at Black v20.8b1 without support for
Python 2.7 or 3.5, so we can turn this check back on. See #4406.

Test Plan:
That CI passes suffices.

wchargin-branch: black-20x-no27-no35

Author: wchargin

.github/workflows/ci.yml

@@ -65,8 +65,7 @@ jobs:
       - run: pip freeze --all
       - name: 'Lint Python code for style with Black'
         # You can run `black .` to fix all Black complaints.
-        # TODO(#4406): Re-enable this check.
-        run: black --check --diff . || true
+        run: black --check --diff .
 
   lint-rust:
     runs-on: ubuntu-16.04