Issue #18609, #18408: _ctypes_add_traceback() now clears the current exception

Issue #18609, #18408: _ctypes_add_traceback() now clears the current exception
while adding the traceback, because it may call indirectly a Python function
and Python functions must not be called with an exception set.

In the case of the issue #18609, _ctypes_add_traceback() called the iso8859-1
decoder which is implemented in Python. Python has a ISO-8859-1 codec
implemented in C. It is not used because PyUnicode_Decode() only uses the C
codec for other names (aliases) of this codec ("latin-1", "latin1" and
"iso-8859-1").