Run isort over GitHub scripts

Because we’ve moved all of our code to the project root and are now
running checks there, we’re catching issues in non-application code,
like the import order of our GitHub scripts. I see no harm in applying
the same formatting requirements to our tools that we apply to the
application, so I went ahead and sorted the imports.

.github/scripts/extract_git_ref.py

@@ -1,8 +1,8 @@
 #!/usr/bin/env python3
 
-from typing import Optional
 import os
 import re
+from typing import Optional
 
 
 def get_branch(gh_event: str, gh_ref: str, gh_head_ref: Optional[str]) -> str:

.github/scripts/extract_git_ref_test.py

@@ -1,5 +1,4 @@
 import pytest
-
 from extract_git_ref import get_branch