Fix __main__.py

Remove an import to sphinx.ext.multiversion and add the correct import

sphinx_multiversion/__main__.py

@@ -1,6 +1,6 @@
 #!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 import sys
-from sphinx.ext.multiversion import main
+from .main import main
 
 sys.exit(main())