While working on a subclass of unittest.TestCase to
support TODO-tests, I found a large number of
__-prefixed attributes in TestCase. The presence of
these attributes (and methods) meant that I had to copy
them over to my new subclass to make python happy.

The attached patch converts these __-prefixed
attributes to _-prefixed attributes, making it much
simpler to subclass TestCase. The patch is against
unittest.py from SVN revision 41775.

Also attached are "before" and "after" versions of my
subclass showing the impact of the patch.