py.path sortlist concept broken on python3

[pytestbot]

• Bitbucket: https://bitbucket.org/pytest-dev/py/issue/25
• Originally reported by: @RonnyPfannschmidt
• Originally created at: 2012-11-06T08:41:30.164

it expects cmp to work for the callable case, which means it wont work on python3 for that case