Spack is internally using a patched version of `argparse` mainly to backport Python 3 functionality
into Python 2. This PR makes it such that for the supported Python 3 versions we use `argparse`
from the standard Python library. This PR has been extracted from #25371 where it was needed
to be able to use recent versions of `pytest`.
* Fixed formatting issues when using a pristine argparse.py
* Fix error message for Python 3.X when missing positional arguments
* Account for the change of API in Python 3.7
* Layout multi-valued args into columns in error messages
* Seamless transition in develop if argparse.pyc is in external
* Be more defensive in case we can't remove the file.
09378f56c0