Be explicit about 3.8

tox.ini

@@ -30,6 +30,7 @@ commands = coverage run --parallel -m pytest {posargs}
 [testenv:py38]
 # Python 3.6+ has a number of compile-time warnings on invalid string escapes.
 # PYTHONWARNINGS=d and --no-compile below make them visible during the Tox run.
+basepython = python3.8
 install_command = pip install --no-compile {opts} {packages}
 setenv =
     PYTHONWARNINGS=d