PyUnicode_FromFormat() crashes in debug build if the argument for %U is not a unicode string.

I thought about using %S. This would correspond possible Python implementation (`'%s=%r' % (key, value)`). But in that case we should increase refcounts of key and value (and maybe even pto->kw) since custom __str__ of the key can remove the value from the dict and make the value variable the hanging reference.

I would prefer never failing __repr__, but other core developers have other opinion. Currently partial.__repr__ raises an exception on deep recursion and when the repr of any argument raises an exception.

Current Michael's patch LGTM, but if his want to suggest the solution with %S it would LGTM too.