Add warning to old/classic class inheritance problem

[Eddy114514]

Python 2 classic classes use depth-first, left-to-right method lookup. Python 3 only has new-style classes, which use C3 MRO. In classic multiple inheritance, this can silently change which base class provides an attribute after migration.

Example:

class A:
    def do_this(self): return "A"

class B(A): pass

class C(A):
    def do_this(self): return "C"

class D(B, C): pass

In Python 2 classic classes, D().do_this() resolves to A.
Under Python 3 / C3 MRO, the same hierarchy resolves to C.

Behavior Difference:
Python 2 classic lookup:
D -> B -> A -> C
Python 3 / C3 lookup:
D -> B -> C -> A
So the same attribute name can resolve to a different provider.

To implement the warning I only consider the classic classes and classes with multiple inheritance. The code compare class MRO against hypothetical C3 MRO, and warn only when a real existing attribute would resolve to a different provider. It also warns when the hierarchy has no valid C3 linearization and would fail in python3.

[ltratt]

Can you fix the PR description (the formatting has gone wrong) and squash the commits to remove the ones that aren't relevant for review (note: I'm not necessarily saying squash down to one commit!).