feat(python): Add enumUnknownDefaultCase support for backward compatibility (with tests)

[wing328]

based on #22037

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[VLeonStieber]

I have a strong interest in getting this feature for the Python generator merged soon.

To me, this line seems to be the cause for the failing tests:

-    'unknown_default_open_api' = 'unknown_default_open_api'
+    UNKNOWN_DEFAULT_OPEN_API  = 'unknown_default_open_api'

Could you please adapt this line and try running the tests again?