TST/FIX: Test for instance rather than string in class representation

Matplotlib has merged SubplotBase into AxesBase now, which makes the
class string representation here GeoAxes now, even though we are
still an _instance_ of GeoAxesSubplot

lib/cartopy/tests/mpl/test_axes.py

@@ -13,7 +13,7 @@
 
 import cartopy.crs as ccrs
 import cartopy.feature as cfeature
-from cartopy.mpl.geoaxes import InterProjectionTransform, GeoAxes
+from cartopy.mpl.geoaxes import InterProjectionTransform, GeoAxes, GeoAxesSubplot
 
 
 class TestNoSpherical:
@@ -119,7 +119,7 @@ def test_single_geometry(self):
 
 def test_geoaxes_subplot():
     ax = plt.subplot(1, 1, 1, projection=ccrs.PlateCarree())
-    assert str(ax.__class__) == "<class 'cartopy.mpl.geoaxes.GeoAxesSubplot'>"
+    assert isinstance(ax, GeoAxesSubplot)
 
 
 @pytest.mark.mpl_image_compare(filename='geoaxes_subslice.png')