Some bugfixes.
Fixes
* Some parts of the library would throw OverflowError on very weird dates/timestamps. Now those are converted to the minimum or maximum accepted date/timestamp. Credits to github user tamarinvs19 in https://github.com/python-caldav/caldav/pull/327
* `DAVResponse.davclient` was always set to None, now it may be set to the `DAVClient` instance. Credits to github user sobolevn in https://github.com/python-caldav/caldav/pull/323
* `DAVResponse.davclient` was always set to None, now it may be set to the `DAVClient` instance. Credits to github user sobolevn in https://github.com/python-caldav/caldav/pull/323
* `examples/sync_examples.py`, the sync token needs to be saved to the database (credits to Savvas Giannoukas)
* Bugfixes in `set_relations`, credits to github user Zocker1999NET in https://github.com/python-caldav/caldav/pull/335 and https://github.com/python-caldav/caldav/pull/333
* Dates that are off the scale are converted to `min_date` and `max_date` (and logging en error) rather than throwing OverflowError, credits to github user tamarinvs19 in https://github.com/python-caldav/caldav/pull/327
* Completing a recurring task with a naïve or floating `DTSTART` would cause a runtime error
* Tests stopped working on python 3.7 and python 3.8 for a while. This was only an issue with libraries used for the testing, and has been mended.
* Bugfix that a 500 internal server error could cause an recursion loop, credits to github user bchardin in https://github.com/python-caldav/caldav/pull/344
* Compatibility-fix for Google calendar, credits to github user e-katov in https://github.com/python-caldav/caldav/pull/344
* Spelling, grammar and removing a useless regexp, credits to github user scop in https://github.com/python-caldav/caldav/pull/337
* Faulty icalendar code caused the code for fixing faulty icalendar code to break, credits to github user yuwash in https://github.com/python-caldav/caldav/pull/347 and https://github.com/python-caldav/caldav/pull/350
* Sorting on uppercase attributes didn't work, ref issue https://github.com/python-caldav/caldav/issues/352 - credits to github user ArtemIsmagilov.
* The sorting algorithm was dependent on vobject library - refactored to use icalendar library instead
* Lots more test code on the sorting, and fixed some corner cases
* Creating a task with a status didn't work