update python version in doc logs

docs/developing-locust.rst

@@ -34,10 +34,10 @@ We use `tox <https://tox.readthedocs.io/en/stable/>`_ to automate tests across m
     $ pip3 install tox
     $ tox
     ...
-    py38: install_deps> python -I -m pip install cryptography mock pyquery retry
-    py38: commands[0]> python3 -m pip install .
+    py39: install_deps> python -I -m pip install cryptography mock pyquery retry
+    py39: commands[0]> python3 -m pip install .
     ...
-    py38: commands[1]> python3 -m unittest discover
+    py39: commands[1]> python3 -m unittest discover
     ...
 
 To only run a specific suite or specific test you can call `pytest <https://docs.pytest.org/>`_ directly: