Fix back-compat import of ext names without dots.

`makeExtension` is the module level function defined in an extension,
not `extendMarkdown`, which is the method you override on the Extension class.
Do'h. Thanks for catching this @mitya57.

markdown/__init__.py

@@ -234,7 +234,7 @@ def build_extension(self, ext_name, configs):
             )
             # For backward compat (until deprecation)
             # check that this is an extension.
-            if ('.' not in ext_name and not (hasattr(module, 'extendMarkdown')
+            if ('.' not in ext_name and not (hasattr(module, 'makeExtension')
                or (class_name and hasattr(module, class_name)))):
                 # We have a name conflict
                 # eg: extensions=['tables'] and PyTables is installed