Fix warnings when run with python -3

[sigfrido]

Fixes the following python 3 compat warnings when we run nose tests with python -3:

• /src/nose/nose/pyversion.py:49: DeprecationWarning: Overriding eq blocks inheritance of hash in 3.x
• /src/nose/unit_tests/mock.py:95: DeprecationWarning: dict.has_key() not supported in 3.x; use the in operator

Here there's also a suggestion to set hash = None if we're sure instances will never be hashed: http://stackoverflow.com/questions/15471333/how-to-eliminate-a-python3-deprecation-warning-for-the-equality-operator.