Use key argument in sort calls

[Martin v. Löwis]

Now that Python 2.4 is the minimum supported version, it is possible to replace all comparison functions with key functions. The attached patch does that.

Doing so has two advantages:

• it may be slightly faster, in particular, if the comparing the key values is faster than calling the cmp function
• it helps moving Django towards Python 3, which doesn't support the compare function anymore.

With the patch, I don't get any additional failures when testing on trunk.

[Martin v. Löwis]

As this is my first Django contribution, I'm not sure what is supposed to happen next. Notice that I don't have commit privileges, so I couldn't check that in myself.

[Karen Tracey]

​http://docs.djangoproject.com/en/dev/internals/contributing/#ticket-triage has a picture and some description of Django's process. You weren't expected to check it in yourself, Alex setting it ready for checkin is just a way to signal he has reviewed it and thinks it's ready.

[Russell Keith-Magee]

(In [13509]) Fixed #12746 -- Updated sorting calls to use 'key' instead of 'cmp'. This will be slightly faster in certain circumstances, but more importantly, is a required step for migration to Python 3. Thanks to Martin van Loewis for the patch.