Deprecated call to `.utcnow()`

[TheToddLuci0]

/home/kali/git/CredMaster/credmaster.py:233: DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC).
  self.start_time = datetime.datetime.utcnow()

[LukeLauterbach]

This is fixed in #82 (which is currently not merged).