PERF: lazify pytz seqToRE call, trims 35ms from import
Shaves on the order of 5% off of import time.

Unrelated docstring fixups, improve typing on parse_timezone_directive and _calc_julian_from_V.

The important thing here is setting the "Z" key dynamically in `__getitem__` instead of in `__init__` (since an instance is created in the module namespace at importt)