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)
How does the __init__ impact import time? Are we instantiating these at import? > How does the init impact import time? Are we instantiating these at import?

Yes, there is an instance of this in the module namespace. lgtm to me Thanks @jbrockmendel  @WillAyd IIRC you made the top-level Panel import lazy in py37.  have you looked at e.g. https://pypi.org/project/modutil/ for doing this in more places?  top-level imports from pandas.io.api look like the next place we can trim Haven't looked a lot at lazy imports, so if you have concrete proposals certainly put them out there.

Just keep in mind as one downside to lazy imports I think you would lose type checking against those libraries, since that import I don't think can be resolved statically