Fixed protocol check failing for inherited class/static methods

[emmanuel-ferdman]

Changes

Fixes #539.

When checking if a class implements a protocol with a classmethod or staticmethod, typeguard only looked at the class's own __dict__ and missed inherited methods. This caused subclasses to fail protocol checks even when they properly inherit the required classmethod from a parent. The fix traverses the MRO to find where the method is actually defined.

Checklist

If this is a user-facing code change, like a bugfix or a new feature, please ensure that
you've fulfilled the following conditions (where applicable):

• You've added tests (in tests/) added which would fail without your patch
• You've updated the documentation (in docs/, in case of behavior changes or new
  features)
• You've added a new changelog entry (in docs/versionhistory.rst).

[coveralls]

coverage: 94.718% (+0.01%) from 94.705%
when pulling 4a0c974 on emmanuel-ferdman:master
into 740c1d1 on agronholm:master.