[Stdlib] Add PythonObject.__contains__

[rd4com]

Simple and small implementation,

Improving the 🐍 Python experience !

Example usage:

x = PythonObject([1,2,3])
if 1 in x: 
   print("1 in x")

That method should be replaced by a c-python function if possible.

[JoeLoser]

Great! Do you mind adding some unit tests please?

[rd4com]

🆕 unit tests

👍 yep it is important, should there be more ?

Python throws an error if the object is not iterable,

But mojo gets an internal error: PyObject_GetAttrString failed from it.

Current PR have a debug_assert when we have the error and returns False.

Is it possible to make __contains__ re-raise the same error as python would have thrown ?