register freeze_time mark with pytest

[dimaqq]

For #13

[alysivji]

LGTM. Straight out of the pytest docs

[dimaqq]

Help needed to pacify continuous testing.

[alysivji]

Looks like the other PRs are failing with the same build... I don't have a Windows machine to test it out, but I did notice that the warning still does show up in the builds:

PytestUnknownMarkWarning: Unknown pytest.mark.freeze_time - is this a typo?  You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/latest/mark.html

Taking a look at the docs again... you have the function in a class, in the docs, the function is in the module. Maybe that's the problem?

[ktosiek]

This was fixed by other PRs (#17 and #20). I'm closing this one.