Replace deepcopy of the Q object

[rrauenza]

[rrauenza]

Still trying to run unit tests locally, hoping marking this PR as ready would trigger some on project side.

[rrauenza]

See https://github.com/django-polymorphic/django-polymorphic/issues/542

[TomHaii]

Can also confirm that is indeed fixing the issue in deepcopying Q objects. (reproduced on Python 3.7)

[alphatownsman]

Is there some unittest can cover the case it fixes?

[rrauenza]

I think if the existing unit tests work, that is sufficient, since it shows the change doesn't break from the current method of copying.

We could assign a non picklable object attr to one of the children to prevent future regression?

[AdamDonna]

+1 to adding a non picklable object attr to prevent future regressions or even an explicit scenario that should break in 3.7 without this fix