Verify that Py 3.9 Dict Union operations are properly supported in type inference

https://www.python.org/dev/peps/pep-0584 added a new syntax for dict union in Python 3.9
I am seeing 3 new opcodes in CPython that look relevant:
https://www.diffchecker.com/kX5h3Cli

SET_UPDATE
DICT_MERGE
DICT_UPDATE

We should add tests for type inference involving these operations and see if we need additional changes to https://github.com/apache/beam/blob/master/sdks/python/apache_beam/typehints/opcodes.py