Target python 3.8 when running mypy.

Ruff and pre-commit hooks already the oldest version to ensure backward compatibility.

Doing the same for mypy ensures no features are used that don't exist in python 3.8.

mypy.ini

@@ -1,5 +1,5 @@
 [mypy]
-python_version = 3.11
+python_version = 3.8
 warn_redundant_casts = True
 disallow_any_generics = true
 disallow_incomplete_defs = true