Convert code to Python 3

[claudep]

This is the first part of the conversion, allowing to launch tests.
The second part should be done by running the tests against a complying
CalDAV/CardDAV server.

[phil-davis]

I ran this just now to try it in CI: https://drone.owncloud.com/owncloud/core/32455/38/9

+ bash apps/dav/tests/ci/caldav/script.sh
[Thu Sep 16 11:20:00 2021] 127.0.0.1:39868 [207]: /remote.php/dav/
[Thu Sep 16 11:20:00 2021] 127.0.0.1:39876 [201]: /remote.php/dav/calendars/user01/synccalendar1/
[Thu Sep 16 11:20:00 2021] 127.0.0.1:39880 [200]: /remote.php/dav/calendars/user01/shared/
[Thu Sep 16 11:20:00 2021] 127.0.0.1:39884 [207]: /remote.php/dav/calendars/user01/shared/
Unable to import Calendar from pycalendar, ignoring.
Starting tests

File 1 of 3

ests.CalDAV.current-user-principal                          
  Suite: Check for the property on /                        

ests.CalDAV.current-user-principal                             [ERROR]

File 2 of 3

ests.CalDAV.sync-report                                        [ERROR]

File 3 of 3

ests.CalDAV.sharing-calendars                               
  Suite: Read-write calendar                                
    Test: 1                                                       [OK]

ests.CalDAV.sharing-calendars                                  [ERROR]

======================================================================
[ERROR]
FATAL ERROR: No module named 'StringIO'

ests.CalDAV.current-user-principal
======================================================================
[ERROR]
FATAL ERROR: must be str, not bytes

ests.CalDAV.sync-report
======================================================================
[ERROR]
FATAL ERROR: No module named 'StringIO'

ests.CalDAV.sharing-calendars
----------------------------------------------------------------------
Ran 4 tests in 0.914s

FAILED (ok=1, ignored=0, failed=0, errors=3)

It runs - that's a good start :) And 1 of 4 tests passed.

I will look and see if there are obvious things to refactor/fix.

[claudep]

Pushed a fix for the missing StringIO messages. For further debugging, it would be nice if the CI could report tracebacks or at least location of errors.