This is the the 3rd-party pytz library (required by the icalendar library), 
used for parsing and generating the text of ICS files.  It's included for 
source-code users who don't want to install it separately, and its license
allows this inclusion.

This library was patched locally in May-2022 for changes in Python.
Search .py files for "FRIGCAL" to find changes applied.

See its README and LICENSE for details of this package, or view at
https://pypi.org/project/pytz/. 