added CalendarAPI with the help of caldav

[bigcat88]

Fixes #120

In future we should use our own DAV stuff that was written for Files already and Objects API from caldav, so we need to implement our custom DAVClient and DAVResponse with compatible format for caldav.

[codecov]

Codecov Report

Merging #136 (97c6358) into main (c12bd13) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #136   +/-   ##
=======================================
  Coverage   99.74%   99.75%           
=======================================
  Files          32       33    +1     
  Lines        2781     2809   +28     
=======================================
+ Hits         2774     2802   +28     
  Misses          7        7           

Files	Coverage Δ
nc_py_api/_session.py	98.47% <100.00%> (ø)
nc_py_api/_version.py	100.00% <100.00%> (ø)
nc_py_api/calendar.py	100.00% <100.00%> (ø)
nc_py_api/nextcloud.py	100.00% <100.00%> (ø)

[bigcat88]

errors should be fixed with nextcloud/app_api@d3d7a60