Fix Python interpreter used by cover3 test env

This has been broken since 7932b15
but it was not noticed because prior to tox 4.2.5 (2023-01-06),
tox used to return exitstatus 0 when all envs were skipped.

tox.ini

@@ -30,7 +30,7 @@ deps =
     pytest-cov
 
 [testenv:cover3]
-basepython = python3.7
+basepython = python3.8
 commands =
     pytest --capture=no --cov=supervisor --cov-report=term-missing --cov-report=xml {posargs}
 deps =