Replace currrent wrappers of Python 2 only PyString_* functions with
better abstractions that handle default "str" (PyStr_*) or byte strings
("str"/"bytes" depending on version, PyStrBytes_*) and adjust all
invocations to work on appropriate string types.
Fixes obvious "attributes typeName and/or value of uno.Enum are not
strings" exceptions with Python 3.

Change-Id: I255dcb1bc198fd7f6a62b83b957901521071a480