Add `co_freevars` to code object

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

Currently, code objects are missing the co_freevars attribute:

def foo(a, b):
    def bar(c, d):
        print(a, b, c, d)

In RustPython:

>>>>> foo.__code__.co_freevars
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'code' object has no attribute 'co_freevars'

While in Python:

>>> foo.__code__.co_freevars
('a', 'b')

looking at the other methods on PyCode should help, the implementation should be similar to others there that return a tuple.