Error if Cython enum is used outside a typed context

[vyasr]

Without a cpdef to generate a corresponding Python enum, cdef enums (and enum classes) are not valid in pure Python context and should be detected accordingly.

I'm not sure if this is the best approach here and would appreciate some feedback, thanks!

Resolves #5638

[shwina]

Note; this closes #5638

[scoder]

Seems reasonable. Not sure whether it's an error or warning, but I agree that there's probably not much sense in such code. That speaks for an error.

In any case, I'll postpone this until after 3.0.2, which urgently fixes major regressions in the 3.0.1 release.

[vyasr]

Seems reasonable. Not sure whether it's an error or warning, but I agree that there's probably not much sense in such code. That speaks for an error.

In any case, I'll postpone this until after 3.0.2, which urgently fixes major regressions in the 3.0.1 release.

Agreed, I think error probably makes the most sense here. It's not like an unused variable or something similar harmless, it's trying to create an object in a context where it shouldn't be possible.

[vyasr]

@scoder do you think this is mergeable, or do you want to see a different approach?

[da-woods]

I think this is fine but just got lost under some more serious bug fixes. I'll merge it to 3.1 now and let @scoder decide whether to backport to 3.0.6.

Thanks for the fix.