natural_earth: countries_50 & land_50 not extending to -90° N

[mathause]

Similarly to #410, natural_earth_v4_1_0.countries_50 and natural_earth_v4_1_0.land_50 (as well as natural_earth_v5_0_0....) don't extend all the way to -90°N, which leads to the southernmost gridcell not being masked. The following don't reach -90°N:

import regionmask

print(regionmask.defined_regions.natural_earth_v4_1_0.countries_50.bounds_global[1])
print(regionmask.defined_regions.natural_earth_v5_0_0.countries_50.bounds_global[1])
print(regionmask.defined_regions.natural_earth_v4_1_0.land_50.bounds_global[1])
print(regionmask.defined_regions.natural_earth_v5_0_0.land_50.bounds_global[1])

# -89.99892578
# -89.99892578
# -89.99892578
# -89.99892578

Resulting in incomplete masks (it only looks so extreme because the grid spacing is 5° - it's just the southernmost gridpoint that does not work):

import regionmask
import matplotlib.pyplot as plt
import numpy as np
import cartopy.crs as ccrs

lon = np.arange(-180, 180, 5)
lat = np.arange(90, -91, -5)


mask = regionmask.defined_regions.natural_earth_v4_1_0.land_50.mask(lon, lat)


ax = regionmask.defined_regions.natural_earth_v4_1_0.land_50.plot(add_coastlines=False, add_label=False, projection=ccrs.Orthographic(central_latitude=-90))
mask.plot(ax=ax, add_colorbar=False, transform=ccrs.PlateCarree())

ax.set_extent([-180, 180, -90, -45], ccrs.PlateCarree())

plt.savefig("land_50_not_southpole.png", dpi=300, bbox_inches="tight")

Note that e.g. land_110 does not have this problem:

import regionmask
print(regionmask.defined_regions.natural_earth_v5_0_0.land_110.bounds_global[1])
# -90.