Fix classmethod wrapping

[theacodes]

Fixes #5789

[tseaver]

@theacodes On second thought, don't we need a testcase which exercises the wrapping of a classmethod?

[theacodes]

@tseaver we have one.

[tseaver]

@theacodes Why wasn't it failing prior to this fix, then?

[theacodes]

A wonderful question. This code is pretty wild to me.

So what was working before was this:

class_instance.class_method(...)

but what wasn't was:

Class.class_method(...)

which lead to #5789. We have a test for the former, but not the latter. I can add one if you want.

[tseaver]

Seems like a test to catch future regressions is the Right Thing(TM).

[theacodes]

#5828