Use a patched argparse only in Python 2.X (#25376)

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.

bin/spack

@@ -28,6 +28,7 @@ exit 1
 from __future__ import print_function
 
 import os
+import os.path
 import sys
 
 min_python3 = (3, 5)
@@ -70,6 +71,28 @@ if "ruamel.yaml" in sys.modules:
 if "ruamel" in sys.modules:
     del sys.modules["ruamel"]
 
+# The following code is here to avoid failures when updating
+# the develop version, due to spurious argparse.pyc files remaining
+# in the libs/spack/external directory, see:
+# https://github.com/spack/spack/pull/25376
+# TODO: Remove in v0.18.0 or later
+try:
+    import argparse
+except ImportError:
+    argparse_pyc = os.path.join(spack_external_libs, 'argparse.pyc')
+    if not os.path.exists(argparse_pyc):
+        raise
+    try:
+        os.remove(argparse_pyc)
+        import argparse  # noqa
+    except Exception:
+        msg = ('The file\n\n\t{0}\n\nis corrupted and cannot be deleted by Spack. '
+               'Either delete it manually or ask some administrator to '
+               'delete it for you.')
+        print(msg.format(argparse_pyc))
+        sys.exit(1)
+
+
 import spack.main  # noqa
 
 # Once we've set up the system path, run the spack main method