Add Type Hints as specified in is-charms-contributing-guide

Author: javierdelapuente

pyproject.toml

@@ -34,10 +34,16 @@ line_length = 99
 profile = "black"
 
 [tool.mypy]
-ignore_missing_imports = true
+check_untyped_defs = true
+disallow_untyped_defs = true
 explicit_package_bases = true
+ignore_missing_imports = true
 namespace_packages = true
 
+[[tool.mypy.overrides]]
+disallow_untyped_defs = false
+module = "tests.*"
+
 [tool.pylint]
 disable = "wrong-import-order"