Use oldest supported Python in mypy pre-commit checks

Follow up to pytest-dev#12744, this ensures type checking works at the oldest Python version supported by pytest.

.pre-commit-config.yaml

@@ -26,6 +26,7 @@ repos:
     -   id: mypy
         files: ^(src/|testing/|scripts/)
         args: []
+        language_version: "3.8"
         additional_dependencies:
           - iniconfig>=1.1.0
           - attrs>=19.2.0