Committed revision 54539.

The committed version issues warnings rather than errors when both methods are overridden, to avoid too much breakage.

The string.py change was necessary to avoid spurious warnings (with no module/lineno!) and breakage of test_subprocess.py.  Something fishy's going on -- is string.Template() used by the warnings module or by site.py???

I'm leaving this bug open but changing the category to Py3k so remind me it needs to be merged and then changed there.