Fix forgotten zone-naive datetime

Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/lib/python3.7/threading.py", line 917, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.7/threading.py", line 865, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/local/lib/python3.7/dist-packages/backupandrestore/utilities/CronScheduler.py", line 247, in __run
    self.jobs[alias].run()
  File "/usr/local/lib/python3.7/dist-packages/backupandrestore/utilities/CronScheduler.py", line 158, in run
    self.jobCallable(*self.jobCallableArgs)
  File "/usr/local/lib/python3.7/dist-packages/backupandrestore/bnr/Postgres.py", line 54, in doBackup
    if self._hasCurrentArchive(db):
  File "/usr/local/lib/python3.7/dist-packages/backupandrestore/bnr/Base.py", line 139, in _hasCurrentArchive
    delta = nowDatetime - backupDatetime
TypeError: can't subtract offset-naive and offset-aware datetimes