fix: specify sphinx version to avoid errors

Author: trixmoe

requirements-sphinx.txt

@@ -11,5 +11,6 @@ typing>=3.5,<=3.5.1
 yapsy==1.11.223
 
 # for sphinx doc only:
+sphinx<1.7
 sphinx-argparse<=0.1.15
 sphinxcontrib-napoleon>=0.2.6,<=0.5.0