unit tests for timezones

[basil96]

Discussion in #27 revealed that our scheduling logic is untested with regard to timezones. We need at least these test cases:

1. WeeklySchedule instance provided with a tzinfo that matches a provided location.

   • expectation: getCivilTwilight should return the appropriate datetime for the location on a given date in the provided tzinfo timezone.

2. WeeklySchedule instance provided with a location and tzinfo=None.

   • expectation: getCivilTwilight should return the appropriate datetime for the provided location on a given date but in the timezone of the caller's OS.

3. WeeklySchedule instance provided with a tzinfo that is different from a provided location.

   • expectation: getCivilTwilight should return the appropriate datetime for the provided location on a given date but in the provided tzinfo timezone.

4. WeeklySchedule instance provided with a location and no tzinfo argument.

   • expectation: getCivilTwilight should return the appropriate datetime for the provided location on a given date but in the UTC timezone.

Choose a few sample dates, locations, and twilight times from timeanddate.com, no fancy web scraping needed.

Bonus: use dates/times/locations where DST is involved. For example, try a location where DST is not practiced (Arizona?) while providing a tzinfo where DST is in effect on the given day.