INCR-416 python3 compatibility

common/lib/xmodule/xmodule/tests/__init__.py

common/lib/xmodule/xmodule/tests/test_annotator_token.py

common/lib/xmodule/xmodule/tests/test_conditional.py

common/lib/xmodule/xmodule/tests/test_error_module.py

common/lib/xmodule/xmodule/tests/test_import_static.py

common/lib/xmodule/xmodule/tests/test_library_content.py

common/lib/xmodule/xmodule/tests/test_sequence.py

common/lib/xmodule/xmodule/tests/test_studio_editable.py

common/lib/xmodule/xmodule/tests/test_validation.py

common/lib/xmodule/xmodule/tests/test_xml_module.py