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.

Newer pytest changed its `--collect-only` output, which then busts the
assumption in our code's regex matching. (Spack's corresponding change a
while back: spack/spack#25371.) In this case,
instead of relying on explicit regex parsing, use pytest's `--quiet`
flag to get the `path::test_name` pair.

Also add in test coverage for the unit-test command.

Newer pytest changed its `--collect-only` output, which then busts the
assumption in our code's regex matching. (Spack's corresponding change a
while back: spack/spack#25371.) In this case,
instead of relying on explicit regex parsing, use pytest's `--quiet`
flag to get the `path::test_name` pair.

Also add in test coverage for the unit-test command.

Newer pytest changed its `--collect-only` output, which then busts the
assumption in our code's regex matching. (Spack's corresponding change a
while back: spack/spack#25371.) In this case,
instead of relying on explicit regex parsing, use pytest's `--quiet`
flag to get the `path::test_name` pair.

Also add in test coverage for the unit-test command.