add mypy tox environment

tox.ini

@@ -37,6 +37,15 @@ commands =
     python -m ruff check --no-fix .
     python -m mypy .
 
+[testenv:mypy]
+; This is a separate tox environment from the 'check' as this one
+; is means to be ran solely in the CI pipelines and used to check the
+; code with mypy using the oldest supported python version.
+description = Run mypy for /src
+deps = -r{toxinidir}/requirements/requirements-mypy.txt
+commands =
+    python -m mypy src
+
 [testenv:builddocs]
 description = Build documentation
 deps = -r{toxinidir}/requirements/requirements-docs.txt