Allow protected-access in class methods

[irgeek]

Steps

• Add yourself to CONTRIBUTORS if you are a new contributor.
• Add a ChangeLog entry describing what your PR does.
• If it's a new feature or an important bug fix, add a What's New entry in doc/whatsnew/<current release.rst>.
• Write a good description on what the PR does.

Description

This is a pretty basic change to allow accessing protected member inside class methods. It performs three checks:

1. Are we in a class method
2. Is the inferred value of the access expression an instance of the class
3. Is the attribute name being accessed an instance or class member of the class

If all three check out, then we allow access to protected members.

I've added tests for the new behaviour, as well as some (potentially redundant) tests to make sure I wasn't picking up attributes of other classes.

I wasn't 100% sure about how to structure the change log entries, but I've added what I thought was correct. If they need to be amended I can certainly do that.

Type of Changes

	Type
✓	✨ New feature

Related Issue

Closes #1159

[coveralls]

Coverage increased (+0.007%) to 91.463% when pulling c4c1087 on irgeek:classmethod-protected-access into 8cab76f on PyCQA:master.

[Pierre-Sassoulas]

Very nice fix, with a lot of test case :) !