Fixed #19560 -- Identified field in warning for naive datetime.

Thanks gcc for the report and vajrasky for the patch.
Fixed #21198 -- Prevented invalid use of @python_2_unicode_compatible.

Thanks jpic for the report and chmodas for working on a patch.

Reverts 2ea80b94. Refs #19362.
Fixed #21256 -- Error in datetime_safe.datetime.combine.