added pip version in spnix

Author: jeniyat

tox.ini

@@ -116,6 +116,7 @@ changedir = doc
 # pip install requirements.txt is separate as RTD does it in separate steps
 # having the requirements.txt installed in deps above results in Double Requirement exception
 # https://github.com/pypa/pip/issues/988
+pip==20.2
 commands =
     pip install --exists-action=w -r requirements.txt
     sphinx-build -T -W -b html -d _build/doctrees-readthedocs -D language=en . _build/html