Replace _autoscaleX_on with get_autoscalex_on() in GeoAxes

[holzschu]

Rationale

Modern implementations of matplotlib have removed the variables _autoscaleXon and _autoscaleYon.
mpl/geoaxes.py references these variables, resulting in an error when running cartopy:

AttributeError: 'GeoAxesSubplot' object has no attribute '_autoscaleXon'

This PR replaces the deprecated variables with the functions get_autoscalex_on() (and set_autoscalex_on()), that are present in matplotlib.

Implications

As far as I can tell, the change was introduced with matplotlib version 3.5.2, released May 3rd, 2022.
Based on what I can see from the matplotlib github site, the change should be compatible with previous versions of matplotlib.

[dopplershift]

The changes look good! Test failures seem unrelated since I see the same in #2056.

[greglucas]

@holzschu , it looks like you still need to sign the CLA: Blocked: CLA needed See https://scitools.org.uk. Submit the form at: https://scitools.org.uk/cla/v4/form

[holzschu]

Thanks for the explanation. I have signed the CLA.

[QuLogic]

Unfortunate this wasn't backported to 0.20.3

[greglucas]

We could cut a 0.21.0 release from main... It looks like it has been about a year since 0.20.0 was released.

[dopplershift]

I don't have a preference, but we need to release something before Matplotlib releases 3.6 with both this and #2071.

[ElmWer]

since matplotlib is updated to 3.6 I get the following error

AttributeError: 'GeoAxesSubplot' object has no attribute '_autoscaleXon'

packages

cartopy 0.20.2
matplotlib-base 3.6.0

code
import matplotlib.pyplot as plt
import cartopy.crs as ccrs
fig = plt.figure()
ax = fig.add_subplot(1, 1, 1, projection=ccrs.Mollweide(central_longitude=0))
ax.stock_img()

Traceback
/tmp/ipykernel_10883/381787420.py in <cell line: 7>()
5 ax = fig.add_subplot(1, 1, 1, projection=ccrs.Mollweide(central_longitude=0))
6 ax.set_global()
----> 7 ax.stock_img()

~/miniconda3/lib/python3.9/site-packages/cartopy/mpl/geoaxes.py in stock_img(self, name)
1062 '50-natural-earth-1-downsampled.png')
1063
-> 1064 return self.imshow(imread(fname), origin='upper',
1065 transform=source_proj,
1066 extent=[-180, 180, -90, 90])

~/miniconda3/lib/python3.9/site-packages/cartopy/mpl/geoaxes.py in wrapper(self, *args, **kwargs)
316
317 kwargs['transform'] = transform
--> 318 return func(self, *args, **kwargs)
319 return wrapper
320

~/miniconda3/lib/python3.9/site-packages/cartopy/mpl/geoaxes.py in imshow(self, img, *args, **kwargs)
1366 'handle a %s in imshow.' % type(transform))
1367
-> 1368 target_extent = self.get_extent(self.projection)
1369 regrid_shape = kwargs.pop('regrid_shape', 750)
1370 regrid_shape = self._regrid_shape_aspect(regrid_shape,

~/miniconda3/lib/python3.9/site-packages/cartopy/mpl/geoaxes.py in get_extent(self, crs)
812
813 """
--> 814 p = self._get_extent_geom(crs)
815 r = p.bounds
816 x1, y1, x2, y2 = r

~/miniconda3/lib/python3.9/site-packages/cartopy/mpl/geoaxes.py in _get_extent_geom(self, crs)
819 def _get_extent_geom(self, crs=None):
820 # Perform the calculations for get_extent(), which just repackages it.
--> 821 with self.hold_limits():
822 if self.get_autoscale_on():
823 self.autoscale_view()

~/miniconda3/lib/python3.9/contextlib.py in enter(self)
117 del self.args, self.kwds, self.func
118 try:
--> 119 return next(self.gen)
120 except StopIteration:
121 raise RuntimeError("generator didn't yield") from None

~/miniconda3/lib/python3.9/site-packages/cartopy/mpl/geoaxes.py in hold_limits(self, hold)
489 view_lim = self.viewLim.frozen().get_points()
490 other = (self.ignore_existing_data_limits,
--> 491 self._autoscaleXon, self._autoscaleYon)
492 try:
493 yield

AttributeError: 'GeoAxesSubplot' object has no attribute '_autoscaleXon'

[greglucas]

@ElmWer, you'll have to upgrade to Cartopy v0.21 to work with Matplotlib 3.6.