Change the format tox action base python

Need to ensure it matches the one expected (best to use latest).

Author: pgjones

tox.ini

@@ -15,7 +15,7 @@ deps = -r{toxinidir}/test-requirements.txt
 commands = pytest --cov=h11 --cov-config=.coveragerc h11
 
 [testenv:format]
-basepython = python3.7
+basepython = python3.8
 deps =
     black
     isort