update min deps

[mathause]

Current situation

Package           Required             Policy               Status
----------------- -------------------- -------------------- ------
python            3.7     (2018-06-27) 3.9     (2020-10-08) <
cartopy           0.17    (2018-11-17) 0.20    (2021-09-17) <
matplotlib-base   3.4     (2021-03-31) 3.5     (2021-11-18) <
numpy             1.17    (2019-08-01) 1.21    (2021-06-23) <
seaborn           0.11    (2020-09-08) 0.11    (2020-09-08) =
setuptools        40.4    (2018-09-18) 40.4    (2018-09-18) =
xarray            0.15    (2020-02-03) 0.20    (2021-11-04) <

at least cartopy for the ax.spine["geo"].

[mathause]

Closed in #50