Remove uses of six.moves that did not cause any type errors

Removal of the remaining imports may require passing "--python-version
3.5" to the mypy command.

tests/test_quickstart.py

@@ -14,7 +14,6 @@ import time
 
 import pytest
 from six import text_type, StringIO
-from six.moves import input
 
 from sphinx import application
 from sphinx.cmd import quickstart as qs