calendar_utils: Replace ics library with icalevnt

.isort.cfg

...ations/0003_remove_calendar_source_calendar_event_hash.py

calendar_utils/migrations/0004_auto_20220505_1228.py

calendar_utils/models.py

calendar_utils/parser.py

district/templates/district/base.html

.../templates/elections2021/elections2021_calendar_page.html

...2021/templates/elections2021/elections2021_home_page.html

region/templates/region/base.html

requirements/base.in

requirements/base.txt

...aign/templates/senat_campaign/calendar_event_snippet.html

shared/templates/shared/calendar_current_events_snippet.html

shared/templates/shared/small_calendar_snippet.html

uniweb/templates/uniweb/blocks/calendar_agenda.html