Issue 3611: in some cases (a __del__ re-raising an exception, when called from inside

Issue 3611: in some cases (a __del__ re-raising an exception, when called from inside
an 'except' clause), the exception __context__ would be reset to None.
This crases the interpreter if this precisely happens inside PyErr_SetObject.

- now the __context__ is properly preserved
- in any case, PyErr_SetObject now saves the current exc_value in a local variable, to
avoid such crashes in the future.

Reviewer: Antoine Pitrou.