LTS backport: Upper bound for pytz dependency

Do we need an upper bound for the pytz dependency? (https://github.com/apache/beam/blob/release-2.5.0/sdks/python/setup.py#L108) We typically have upper bounds, in order to avoid future breakages due to a possibility of breaking/backward incompatible change of that depepdency.

Good practice is to upper bound either at known version, or next major version. Do we need an exception for pytz because it does not seem to be following semantic versioning?

cc: Yifan Zou Is this something dependency notifier can warn on? Dependency without upper version bounds.