Rotated coordinates are not shown correctly with plot="poly"

[Chilipp]

Code Sample, a copy-pastable example if possible

take psy-views rotated-pole-test.nc file for instance, and plot it via

psy.plot.mapplot("rotated-pole-test.nc", plot="poly", name="t2m")

Problem description

The grid cell boundaries are not transformed correctly, what you get is

Expected Output

What you should get is

Output of psyplot -aV

Details

psy_maps.plugin:
  requirements:
    cartopy: 0.17.0
  version: 1.3.0+5.g0fc5e8c
psy_reg.plugin:
  requirements:
    scipy: 1.4.1
    statsmodels: 0.11.1
  version: 1.3.0+2.gd87aa82
psy_simple.plugin:
  version: 1.3.0+4.g9388e25
psy_strat.plugin:
  version: 0.1.0
psyplot:
  requirements:
    matplotlib: 3.2.1
    numpy: 1.18.4
    pandas: 1.0.3
    python: 3.7.6 | packaged by conda-forge | (default, Mar 23 2020, 23:03:20)  [GCC
      7.3.0]
    xarray: 0.15.1
  version: 1.3.0+10.g867092b
psyplot_gui:
  requirements:
    pyqt: 5.12.3
    qt: 5.12.5
    qtconsole: 4.7.4
  version: 1.3.0+3.g7df7d45
psyplot_test.plugin:
  version: 1.0.0