Add `in` operator method to `PyAny`

[davidhewitt]

We don't have a method for x in foo on PyAny.

We have PySequence::contains - probably this just wants to be copied as PyAny::contains, with some tests to check this does behave like Python's in operator.

As per #1844 (comment)

[aganders3]

I'm interested to take this on!

For design purposes, the in keyword means two things in Python. This was kind of covered in the linked PR where there was discussion of contains_key but I think it was kept out. The idea here then is to use PySequence_Contains for PyAny::contains, right? Is there any need to do a check first?

[davidhewitt]

Great, thank you!

The idea here then is to use PySequence_Contains for PyAny::contains, right? Is there any need to do a check first?

Yes, I think probably calling PySequence_Contains is all the implementation needs. A few tests should be able to confirm that it behaves as expected.