Add Mapping::contains

[Gobot1234]

Pretty self explainitory.

This function like the collections.abc.Mapping equivalent cannot fail (https://github.com/python/cpython/blob/8fb36494501aad5b0c1d34311c9743c60bb9926c/Lib/_collections_abc.py#L823-L829).

[davidhewitt]

The analogy with collections.abc.Mapping is reasonable, however it makes me uncomfortable too. This isn't equivalent to the in operator; we just merged #2115 which added PyAny::contains. Given that, do we really need to add this too?

In particular, the thing I don't like about PyMapping_HasKey (and particular wrapping it in a method called contains) is that it looks like it should call __contains__, but it does not.

[Gobot1234]

Ok fair enough, what about just making it call PyAny::contains?

[davidhewitt]

I'd prefer that :)

[davidhewitt]

Thanks!