In Python 2, a context manager's __exit__() leaves sys.exc_info with the exception values even when it was supposed
to suppress the exception, so we explicitly call sys.exc_clear() which removes the traceback and allow the object
to be released.
Also updated the test to not depend on the immediate destruction of the object but instead to ensure it is not being
tracked as a cyclic reference.
Fix#1965
We used to have this when we where patching the real Python
AssertionError for use with reinterpret, but reinterpret is now
gone so we no longer need this as it is not used by rewrite.
Works in a similar manner to `raises`, but for warnings instead
of exceptions. Also refactored `recwarn.py` so that all the
warning recording and checking use the same core code.
If the type is not checked then an incomprehensible error will occur
later. This enforces the type and raies the same exception/msg as
CPython does in that case.
Docstring unmodified, just re-justified for pep8 compat.