DOC: fix ImportErrors in apigen.py for Python 3

Author: grlee77

doc/tools/apigen.py

@@ -207,7 +207,11 @@ 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])
+        try:
+            mod = __import__(uri, fromlist=[uri])
+        except ImportError:
+            # fix ImportError for submodules in Python 3
+            mod = __import__(uri, fromlist=[])
         # find all public objects in the module.
         obj_strs = [obj for obj in dir(mod) if not obj.startswith('_')]
         functions = []