Fix pickling when pytest is loaded.

Older versions of pytest will add a "None" module to sys.modules.

cloudpickle/cloudpickle.py

@@ -384,7 +384,8 @@ def _save_subimports(self, code, top_level_dependencies):
                 # check if the package has any currently loaded sub-imports
                 prefix = x.__name__ + '.'
                 for name, module in sys.modules.items():
-                    if name.startswith(prefix):
+                    # Older versions of pytest will add a "None" module to sys.modules.
+                    if name is not None and name.startswith(prefix):
                         # check whether the function can address the sub-module
                         tokens = set(name[len(prefix):].split('.'))
                         if not tokens - set(code.co_names):