Add `co_stacksize` to code objects.

Opening this separately from #1838 since it also seems like a good first issue.

Currently, code objects are missing the co_stacksize attribute:

def foo(a, b): pass

In RustPython:

>>>>> foo.__code__.co_stacksize
AttributeError: 'code' object has no attribute 'co_stacksize'

While in Python:

>>> foo.__code__.co_stacksize
1

looking at the other methods on PyCode should help, the implementation should be similar to those (also related #4502 #4501)