Migrate away from pytz

[mfeit-internet2]

Seen on D12:

/usr/lib/python3/dist-packages/pscheduler/iso8601.py:33: PytzUsageWarning: The localize method is no longer necessary, as this time zone supports the fold attribute (PEP 495). For more details on migrating to a PEP 495-compliant implementation, see https://pytz-deprecation-shim.readthedocs.io/en/latest/migration.html
  parsed = get_localzone().localize(parsed)

Per the link above, we need to migrate away from pytz. It is used in these places:

./pscheduler-core/pscheduler-core/plot-schedule:import pytz
./pscheduler-core/pscheduler-core/plot-schedule:            absolute = pytz.utc.localize(absolute)

./pscheduler-core/pscheduler-core/schedule:import pytz
./pscheduler-core/pscheduler-core/schedule:            absolute = pytz.utc.localize(absolute)

./pscheduler-server/pscheduler-server/api-server/pschedulerapiserver/admin.py:import pytz

./pscheduler-tool-powstream/powstream/run:import pytz
./pscheduler-tool-powstream/powstream/run:start_time = datetime.datetime.utcnow().replace(tzinfo=pytz.utc)
./pscheduler-tool-powstream/powstream/powstream_utils.py:import pytz
./pscheduler-tool-powstream/powstream/powstream_utils.py:    now = datetime.datetime.utcnow().replace(tzinfo=pytz.utc)

./python-pscheduler/pscheduler/pscheduler/pstime.py:import pytz
./python-pscheduler/pscheduler/pscheduler/pstime.py:    return datetime.datetime.fromtimestamp(0, pytz.utc)

./python-pscheduler/pscheduler/pscheduler/clockstate.py:import pytz
./python-pscheduler/pscheduler/pscheduler/clockstate.py:    time_here = pytz.utc.localize(utc).astimezone(local_tz)

./python-pscheduler/pscheduler/setup.py:        'pytz >= 2016.6',