Docs: tox requires Python 3.9+ to run

Author: robsdedude

docs/user_guide.rst

@@ -361,7 +361,7 @@ Main features
 
 * ``plugin system`` to modify tox execution with simple hooks.
 * uses :pypi:`pip` and :pypi:`virtualenv` by default. Support for plugins replacing it with their own.
-* **cross-Python compatible**: tox requires CPython 3.7 and higher, but it can create environments 2.7 or later.
+* **cross-Python compatible**: tox requires CPython 3.9 and higher, but it can create environments 2.7 or later.
   Special configuration might be required: :ref:`eol-version-support`.
 * **cross-platform**: Windows, macOS and Unix style environments
 * **full interoperability with devpi**: is integrated with and is used for testing in the :pypi:`devpi` system, a