Test for PyData.ClassName currently fails for py3_pickle3 zkind:

    === RUN   TestPyClassName/py3_pickle3
        pydata_test.go:50: class name for "\x80\x03X\x18\x00\x00\x00ZODB.tests.testSerializeq\x00X\x13\x00\x00\x00ClassWithoutNewargsq\x01\x86q\x02N\x86q\x03.":
            have: "?.?"
            want: "ZODB.tests.testSerialize.ClassWithoutNewargs"
        pydata_test.go:50: class name for "\x80\x03X\x18\x00\x00\x00ZODB.tests.testSerializeq\x00X\x10\x00\x00\x00ClassWithNewargsq\x01\x86q\x02K\x01\x85q\x03\x86q\x04.":
            have: "?.?"
            want: "ZODB.tests.testSerialize.ClassWithNewargs"

This happens because classname decoder was expecting class and module
names to come as py2 bytestrings, while py3 emits them as unicode.

-> Fix it by accepting both py2 bytestring and unicode for class and
module names.