Support polar axes labels with cartopy

Description

Now, cartopy has supported the axes labels for polar projection.

But, proplot doesn’t work with the same newest version of cartopy.

Steps to reproduce

import proplot as plot
fig, axs = plot.subplots(proj='npstere')
axs.format(coast=True, latlines=10, lonlines=30, lonlabels=True, labels=True, boundinglat=60)

Expected behavior (matplotlib with cartopy)

Actual behavior: (proplot with cartopy)

Equivalent steps in matplotlib

import matplotlib.pyplot as plt
import cartopy.crs as ccrs

ax = plt.subplot(projection=ccrs.NorthPolarStereo())
ax.set_extent([-180,180,90,60], ccrs.PlateCarree())
ax.coastlines()
ax.gridlines(draw_labels=True)

Proplot version

0.6.4

[zxdawn]

@lukelbd Great! Thanks for coming back to polish proplot 😉 Hope your Ph.D. career goes well.

[Thomas-Moore-Creative]

indeed, as said by @zxdawn - great to see you back @lukelbd, keeping proplot shiny!