This is fixed now:

Python 3.10.0a2+ (heads/bpo-42184:f3cb814315, Nov  7 2020, 00:31:51) [MSC v.1916 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import argparse
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('foo', nargs='1')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\User\src\cpython\lib\argparse.py", line 1430, in add_argument
    self._get_formatter()._format_args(action, None)
  File "C:\Users\User\src\cpython\lib\argparse.py", line 611, in _format_args
    raise ValueError("invalid nargs value") from None
ValueError: invalid nargs value