Create timezone.dst() with return None for timetuple() fix

[tekktrik]

Fixes #18, which was failing because timezone didn't have a default dst() to use if a timezone was provided. New behavior based on issue test code:

>>> import adafruit_datetime
>>> dt = adafruit_datetime.datetime(2022, 7, 13,  13, 30, 15 , 0, adafruit_datetime.timezone.utc)
>>> dt.ctime()
'Wed Jul 13 13:30:15 2022'
>>> dt.timetuple()
time.struct_time(tm_year=2022, tm_mon=7, tm_mday=13, tm_hour=13, tm_min=30, tm_sec=15, tm_wday=2, tm_yday=194, tm_isdst=-1)

[FoamyGuy]

Looks good to me. Tested successfully with Feather TFT.
Thanks for the fix @tekktrik