Rename rst2html.py to rst2html5 - new render backend in docutils

use Python 3.11 for this b/c different versions of docutils get
installed for different Python versions and the commands and their names
seem to be different across versions.

Author: atodorov

tox.ini

@@ -37,10 +37,11 @@ commands=
     coverage html
 
 [testenv:readme]
+basepython = python3.11
 skip_install = True
 deps =
     docutils
     Pygments
 commands =
-    rst2html.py --report=info --halt=warning README.rst /dev/null
-    rst2html.py --report=info --halt=warning CHANGELOG.rst /dev/null
+    rst2html5 --report=info --halt=warning README.rst /dev/null
+    rst2html5 --report=info --halt=warning CHANGELOG.rst /dev/null