ci: Ensure mypy is run with Python 3.8

Because mypy needs to parse not just our code but also the code of the
libraries our code depends upon, we can't reliably run mypy in a Python
3.10 environment and tell it to check our code for Python 3.8 type
correctness, because we might install a version of one of our
dependencies with Python 3.10 syntax in it that mypy will fail to parse.

Signed-off-by: Matt Wozniski <mwozniski@bloomberg.net>

Author: godlygeek

tox.ini

@@ -48,7 +48,7 @@ commands =
 
 [testenv:lint]
 description = lint code in {basepython}
-basepython = python3.10
+basepython = python3.8
 extras =
     lint
 commands =