python3.7 installation failing - so why?

[Frank Miles]

I have a Debian/Linux machine that I just upgraded to the newer "testing" 
distribution.  I'd done that earlier to another machine and all went 
well.  With the latest machine, python2 is OK but python3 can barely run 
at all.  For example:

$ 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'
>>> 

Question: how can I determine what has gone wrong?

Reinstalling most of the packages hasn't done any good.  I did a similar 
upgrade to another machine -- and it doesn't have this problem.  Running
md5sum on the above modules as well as /usr/bin/python3.7 generates the
same numbers (comparing working and nonfunctional machines).

Any clues/hints/ideas would be appreciated.  There's a whole lot broken 
on this machine with python3 inoperable :(

  -F