The non-standard `contains` and `in` membership operators currently work for any [sequence](https://docs.python.org/3/library/collections.abc.html#collections.abc.Sequence). https://github.com/jg-rp/python-jsonpath/blob/880c12339f4f3909eb81e2b4d609244f75b56d39/jsonpath/env.py#L551-L554 We should change that so they work for any [container](https://docs.python.org/3/library/collections.abc.html#collections.abc.Container) (any [mapping](https://docs.python.org/3/library/collections.abc.html#collections.abc.Mapping) is a container).
