v2.0.1 reverts the requests->niquests change. See python-caldav#530

Here are the most important changes in 2.0:

* Version 2.0 drops support for old python versions and replaces requests 2.x with niquests 3.x, a fork of requests.
* Major overhaul of the documentation
* Support for reading configuration from a config file or environmental variables - I didn't consider that to be within the scope of the caldav library, but why not - why should every application reinvent some configuration file format, and if an end-user have several applications based on python-caldav, why should he need to configure the caldav credentials explicitly for each of them?
* New method `davclient.principals()` to search for other principals on the server - and from there it's possible to do calendar searches and probe what calendars one have access to.  If the server will allow it.

Starting point for development on an upcoming 2.0-release

Version 1.6.0 will be the last minor release before 2.0. The scheduling

support has been fixed up a bit, and saving a single recurrence does
what it should do, rather than messing up the whole series.

Version 1.5 comes with support for alarms (searching for alarms if th…

…e server permits and easy interface for adding alamrs when creating events), lots of workarounds and fixes ensuring compatibility with various servers, refactored some code, and done some preparations for the upcoming server compatibility hints project.

* Lots of work lifting the project up to more modern standards and im…

…proving code, thanks to Georges Toth (github @sim0nx), Matthias Urlichs (github @smurfix) and @ArtemIsmagilov.  While this shouldn't matter for existing users, it will make the library more future-proof.

* Quite long lists of fixes, improvements and some few changes, nothing big, main focus is on ensuring compatibility with as many server implementations as possible.  See the CHANGELOG for details.

Version 1.3.9 contains various bugfixes, refer to the CHANGELOG.md fo…

…r details

Version 1.3.8

Various bugfixes.

Refer to the CHANGELOG.md for a full list

Version 1.3.7

Some bugfixes, full list in the CHANGELOG.

Very minor test fix