tests: run commit-message with Python 3.8 instead of Python 3.7.17

Pywikibot package requires Python>=3.8.
Keeping basepython = python3 isntalls Python 3.7.17
and the commit-message-validator will fail with ERROR:
Package 'pywikibot' requires a different Python: 3.7.17 not in '>=3.8.0'

Change-Id: Ie1976c1fe5941913eeb30ec2b0f1d71d676401bd

tox.ini

@@ -70,7 +70,7 @@ commands =
     pytest --mypy -m mypy pywikibot
 
 [testenv:commit-message]
-basepython = python3
+basepython = python3.8
 deps = commit-message-validator
 commands = commit-message-validator