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python3.7 disfunctional (was: argparse missing from libpython3.7-stdlib ?)

I just upgraded my desktop from stable/stretch to testing/buster.
I'd earlier done the same to a laptop without any problems.
On the desktop, I get the following strange traceback :

$ python3
Python 3.7.2+ (default, Feb  2 2019, 14:31:48) 
[GCC 8.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> help()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.7/_sitebuiltins.py", line 102, in __call__
    import pydoc
  File "/usr/lib/python3.7/pydoc.py", line 66, in <module>
    import inspect
  File "/usr/lib/python3.7/inspect.py", line 40, in <module>
    import linecache
  File "/usr/lib/python3.7/linecache.py", line 11, in <module>
    import tokenize
  File "/usr/lib/python3.7/tokenize.py", line 33, in <module>
    import re
  File "/usr/lib/python3.7/re.py", line 143, in <module>
    class RegexFlag(enum.IntFlag):
AttributeError: module 'enum' has no attribute 'IntFlag'

I've tried reinstalling python3.7 and its library packages
without effect.

Any hints on how I might find what's causing this pathological
behavior?  TIA!