failing cftime plot tests

[keewis]

This was reported in #5077 and seems to be an upstream issue (see #5077 (comment)):

FAILED xarray/tests/test_plot.py::TestCFDatetimePlot::test_cfdatetime_line_plot
FAILED xarray/tests/test_plot.py::TestCFDatetimePlot::test_cfdatetime_pcolormesh_plot
FAILED xarray/tests/test_plot.py::TestCFDatetimePlot::test_cfdatetime_contour_plot

In order to get the upsteam-dev CI open new issues for other failing tests on upstream-dev I've xfail'ed them in #5343. This should be undone once it is fixed in either cftime or nc-time-axis.

[spencerkclark]

Thanks @keewis; I agree it's good to just xfail them for now.

[aulemahal]

FYI, I opened SciTools/nc-time-axis#59 to update nc-time-axis. Waiting for maintainers to review.