refactor: improve types in python extension

[romanc]

Description

The pre-release checklist highlighted a typing issue in python_extension.py where a while condition wouldn't use is (not) None and thus mypy couldn't safely derive the type and thus the last return was deemed unreachable code (which isn't true). This is a quick PR to improve the types such that mypy is happy.

How has this been tested?

Tested locally and as part of NOAA-GFDL/pace#175 (which is currently pointing NDSL to this branch).

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas: N/A
• I have made corresponding changes to the documentation (e.g. add new modules to docs/docstrings/): N/A
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included: N/A