use pytest instead of nosetests

[azmeuk]

This patch:

• installs pytest and pytest-cov to run the tests
• configure tox so it runs with pytest
• replace nose style assertions assert_equal, assert_not_equal and assert_raises with their pytest equivalents assert and with pytest.raises
• fixes a couple of warnings raised by pytest about regex string missing r flag

Before nose can be totally removed, it is needed to replace the SkipTest exception with the pytest @pytest.mark.skip or @pytest.mark.skipif decorator equivalent. https://github.com/python-caldav/caldav/blob/6b791de7ccae48901ada8318158ae824f8f36982/tests/test_caldav.py#L25 Though it is not as simple as I expected so I havn't implement this in this patch.

Fixes partially #196

[tobixen]

Thanks a lot.

I wonder if the xandikos<0.2.4 still makes sense. As far as I remember, I was spending quite some efforts mending the test framework to support newer versions of xandikos. For all I know, support for elder xandikos may even be broken now. (I don't have time looking into it right now)

[tobixen]

Four checks passed. I guess that means the old xandikos version still works :-)