Use --strict mypy by default

Author: hf-kklein

Diff for: tox.ini

@@ -37,8 +37,8 @@ deps =
     {[testenv:tests]deps}
     -r dev_requirements/requirements-type_check.txt
 commands =
-    mypy --show-error-codes src/mypackage
-    mypy --show-error-codes unittests
+    mypy --show-error-codes src/mypackage --strict
+    mypy --show-error-codes unittests --strict
     # add single files (ending with .py) or packages here
 
 [testenv:spell_check]