Issue #29507: Update test_exceptions

test_unraisable() of test_exceptions expects that PyErr_WriteUnraisable(method)
fails on repr(method).

Before the previous change (7b8df4a5d81d), slot_tp_finalize() called
PyErr_WriteUnraisable() with a PyMethodObject. In this case, repr(method) calls
repr(self) which is BrokenRepr.__repr__() and the calls raises a new exception.

After the previous change, slot_tp_finalize() uses an unbound method: repr() is
called on a regular __del__() method which doesn't call repr(self). repr()
doesn't fail anymore.

PyErr_WriteUnraisable() doesn't call __repr__() anymore, so remove BrokenRepr
unit test.