No more string exceptions for 2.6

I just checked in the next step in PEP 352 for 2.6: no more string exceptions! You now get a TypeError if you try to raise a string exception. There is also a DeprecationWarning raised if you try to catch a string exception.

People have been talking about their removal for a long time (before I got involved in python-dev). Feels good to have made it happen.