I'm attaching a patch that works without changing the recursion limit, and adds some tests for pathological cases.

Instead, PyErr_GivenExceptionMatches is changed so that if an exception was previously set, it is replaced by an exception that PyObject_IsSubclass raises.  In that way recursion errors should be propagated properly.

In exception matching, this means that exceptions (including recursion errors) from PyObject_IsSubclass are ignored.  There is already an explicit test for this behavior in test_exceptions.

This behavior *could* be changed if intended by introducing a variant of PyErr_GivenExceptionMatches that can set an exception even if none was set before, and calling that in cmp_outcome in ceval.