bpo-43874: fix argparse sub parser error when sub parser has no "name"

[terencehonles]

This change updates the following crash to instead produce the error message "%(prog)s: error: the following arguments are required: command".

Previously, when creating a simple required sub parser the following will result in an error instead of an error message:

>>> from argparse import ArgumentParser
>>>
>>> parser = ArgumentParser()
>>> subparsers = parser.add_subparsers(required=True)
>>> subparsers.add_parser('one')
>>> subparsers.add_parser('two')
>>>
>>> parser.parse_args([])

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python3.8/argparse.py", line 1768, in parse_args
    args, argv = self.parse_known_args(args, namespace)
  File "/usr/lib64/python3.8/argparse.py", line 1800, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "/usr/lib64/python3.8/argparse.py", line 2035, in _parse_known_args
    ', '.join(required_actions))
TypeError: sequence item 0: expected str instance, NoneType found

https://bugs.python.org/issue43874

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[terencehonles]

Looks like I had(?) to check if the CLA had been processed in order to have the bot update the labels.

[github-actions]

This PR is stale because it has been open for 30 days with no activity.

[terencehonles]

bump

[terencehonles]

I believe this is a duplicate of #3680, and it looks like that one took awhile to go through