build: not sure when rst2html.py became rst2html

Author: nedbat

tox.ini

@@ -71,7 +71,7 @@ deps =
     sphinx
 
 commands =
-    rst2html.py --strict README.rst /tmp/django_coverage_plugin_README.html
+    rst2html --strict README.rst /tmp/django_coverage_plugin_README.html
 
 [gh-actions]
 python =