[BEAM-14172] Update tox.ini for pydocs (#17176)

Author: aaltay

sdks/python/tox.ini

@@ -143,6 +143,7 @@ deps =
   Sphinx==1.8.5
   sphinx_rtd_theme==0.4.3
   docutils<0.18
+  Jinja2==3.0.3 # TODO(BEAM-14172): Sphinx version is too old.
 commands =
   time {toxinidir}/scripts/generate_pydoc.sh