defer.py: Remove py3k deprecation warnngs when running twisted/test

[drtimcouper]

To demonstrate success:

1. Run trial twisted/test and save the deprecation warnings in a file (so that's 2>1)
2. Apply the patch
3. Run trial twisted/test again to another file, and diff the two.

There should be no defer.py references in the second file which refer to py3k deprecations

[drtimcouper]

version 2 with a missed deprecation included

[exarkun]

1. See #4065 for the discussion about how to deal with callable
2. Converting to __eq__ from __cmp__ shouldn't throw away the nice docstring about how comparison is defined.
3. __cmp__ gives us == and !-, so I think we need __ne__ as well. It might make sense to use twisted.python.util.FancyEqMixin.
4. Adding __hash__ to fix the 3k warning is good, but it looks like that behavior is also untested. We should add tests for the behavior, and we should probably also implement hashability so that it makes sense related to the comparison implementation.

Thanks!