tox: bump Python minimal version to 3.9

Versions 3.8 and downwards are end of life

Link: https://devguide.python.org/versions/
Signed-off-by: Ariel Otilibili <otilibil@eurecom.fr>

Author: ariel-anieli

tox.ini

@@ -2,7 +2,7 @@
 # SPDX-License-Identifier: BSD-3-Clause
 
 [tox]
-envlist = format,lint,py{36,37,38,39,310,311,312,py3},coverage
+envlist = format,lint,py{39,310,311,312,313,py3},coverage
 skip_missing_interpreters = true
 isolated_build = true
 distdir = {toxinidir}/dist