Added requirements.txt to doc build environment (#14171)

For the API documentation as sphinx autogenerates the
api docs from the code it needs the dependent Python
packages. This change adds those to the tox docs build
environment.

py/tox.ini

@@ -6,6 +6,7 @@ skip_install = true
 deps =
   Jinja2==3.0.3
   Sphinx==1.8.2
+  -r requirements.txt
 
 commands = sphinx-build -b html -d ../build/docs/doctrees docs/source ../build/docs/api/py {posargs}