snap build installing old pypi version of argparse

When building for core20 the python extension seems to be loading pypi modules not in requirements.txt. The snapcraft.yaml attached shows the fix (lines 57-60) for the old argparse in pypi being substituted for the newer argparse in the py38. Without the uninstall, argparse is included from pypi, and that version is way too old for us to use.

This is suspicious because normally pip would not install anything not in requirements.txt, but in this case argparse from pypi is installed. Normal operation of pip install does not install extras. Do I need to look for more trouble spots? What else might be installed in error?