overides type check should allow methods to return None (or not)

[Christian-B]

In mypy declaring a None return is Optional.
An exception is that if there are no prams a None declaration is needed to trigger type checking

We had choose the style of not declaring the None return

Previously the overrides check had forced both overrider and overwriten to either both or neither to declare the None return

This pr allows one to declare a None return and the other not to.

[coveralls]

coverage: 88.275% (+0.1%) from 88.179%
when pulling caaf714 on allow_none
into e13f213 on master.