Rename Method refactoring is allowed in methods of the descriptor protocol

[jonhnanthan]

Steps to reproduce the behavior:

1. Code before refactoring:

class Descriptor:
    def __get__(self, instance, owner):
        return 40

2. Apply the Rename Method refactoring to the method __get__.

3. Expected code after refactoring: the same as the original. It would be nice if Rope could emit a warning that applying the refactoring to this type of class is not allowed.

Rope produces the result below:

class Descriptor:
    def value(self, instance, owner):
        return 40