Fix BaseDocument._from_son() deepcopy failed with compiled regex

[StoneMoe]

Fix for issue #2429

[bagerard]

Hi @StoneMoe , thanks for your contribution. It looks great, I only have a minor few comments. Please check the comments and let me know if there are additional questions.

[StoneMoe]

Hi, thanks for the reviewing, I added more tests and changed approach for this fix based on that.

but there are some question might need more discussion:

1. deepcopy() won't copy class-level attributes added by __new__, which is instance._fields in this case. which resulting in the test you request to add won't pass in the first place, is this expected or another bug for mongoengine?
2. deepcopy() works for _fields attribute of the instance returned by _from_son(), but not work for the objects returned from queryset

[bagerard]

sorry I lost track of this. I tested the initial issues on latest master and it's not failing anymore so I guess something got merged since this PR got created. I'm including the test cases in #2865 to avoid regression but I'm closing this one as I prefer to avoid too much hacks on deepcopy internals unless absolutly necessary