Matplotlib-3.7 has merged SubplotBase into AxesBase now, which makes the
class string representation here GeoAxes now, even though we are
still an _instance_ of GeoAxesSubplot

Backported from https://github.com/SciTools/cartopy/commit/6b4572ba1a8a877f28e25dfe9559c14b7a565958?diff=unified
diff --git a/lib/cartopy/tests/mpl/test_axes.py b/lib/cartopy/tests/mpl/test_axes.py
index eaf5904..d4e37a1 100644
--- a/lib/cartopy/tests/mpl/test_axes.py
+++ b/lib/cartopy/tests/mpl/test_axes.py
@@ -13,7 +13,8 @@ import pytest
 
 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 +120,7 @@ class Test_Axes_add_geometries:
 
 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')