v0.13.0 - remove pytz dependency
Remove pytz dependency (#10) * Remove pytz dependency `datetime.timezone.utc` exists since at least 3.5, so guaranteed to be in 3.7 Also removed `dateutil.parser` from tests in favor of direct datetime creation (tried to remove whole dateutil, but `datetime.fromisoformat` isn't good enough, and dragging in `ciso8601` is a bit of overkill.