When spying on methods, we replaced the method with a new instance, with

the intended purpose of preventing spies on an instance method from

impacting other instance methods. This caused subtle problems for

classmethods, which caused state to not be tracked quite right and to

leak.
This change fixes this, ensuring we don't override any classmethods.

Unit tests pass on all supported Python versions.
The breakages I encountered spying on classmethods in some Djblets

unit tests went away.