partialmethod re-implementation for py2.7 does not work

[kinverarity1]

The implementation of partialmethod in utils.py (used when python 2.7 is present) doesn't work, because it is used on a @classmethod, which is not callable (ref python 3.4+ implementation of partialmethod: https://github.com/python/cpython/blob/e56975351bc2c574c728f738e88effba7116279f/Lib/functools.py#L331)

I'll try to work out a solution.