DOC: fix ImportErrors in apigen.py for Python 3

The fix was copied from scikit-image's version of apigen.py

Author: grlee77

doc/tools/apigen.py

@@ -207,7 +207,7 @@ def _parse_module_with_import(self, uri):
         classes : list of str
             A list of (public) class names in the module.
         """
-        mod = __import__(uri, fromlist=[uri])
+        mod = __import__(uri, fromlist=[uri.split('.')[-1]])
         # find all public objects in the module.
         obj_strs = [obj for obj in dir(mod) if not obj.startswith('_')]
         functions = []