Type hints are not very useful if they are not checked.
Breaking change: Invalid.path and Invalid.error_message are no longer writable (properties). Because they are propertis in subclass MultipleInvalid and subclass can't override writable as read only.
Fix definition of Schemable. Extra is not a type but Callable and Callable is already there. For NoneType just None should be used.
Add mypy to run with tox.
Fix flake8 warnings regarding using isinstance instead of direct comparison.
Type hints are not very useful if they are not checked.
Invalid.pathandInvalid.error_messageare no longer writable (properties). Because they are propertis in subclassMultipleInvalidand subclass can't override writable as read only.Schemable.Extrais not a type butCallableandCallableis already there. ForNoneTypejustNoneshould be used.isinstanceinstead of direct comparison.