Try/except import of Axes3D

On some installs, the `mpl_toolkits` namespace package gets an old
version which uses deprecated (And removed) code from the main library.

On such the import of Axes3D will error with an `ImportError`.
This prevents users from using any of `matplotlib`, since it is imported
unconditionally by default.

This just try/excepts the imports (and adjusts the registration code
accordingly) to allow users to continue using matplotlib (though not 3D
and possibly not other mpl_toolkits) even with older installs occluding
the mpl_toolkits.

Author: ksunden

lib/matplotlib/projections/__init__.py

@@ -55,7 +55,11 @@
 from .. import axes, _docstring
 from .geo import AitoffAxes, HammerAxes, LambertAxes, MollweideAxes
 from .polar import PolarAxes
-from mpl_toolkits.mplot3d import Axes3D
+
+try:
+    from mpl_toolkits.mplot3d import Axes3D
+except ImportError:
+    Axes3D = None
 
 
 class ProjectionRegistry:
@@ -87,8 +91,12 @@ def get_projection_names(self):
     HammerAxes,
     LambertAxes,
     MollweideAxes,
-    Axes3D,
 )
+if Axes3D is not None:
+    projection_registry.register(Axes3D)
+else:
+    # remove from namespace if not importable
+    del Axes3D
 
 
 def register_projection(cls):