cext: use modern buffer protocol in patches() PyObject_AsCharBuffer() is part of the "Old Buffer Protocol," which has been deprecated for years. Let's port away from it. PyBuffer_GetBuffer() must be paired with PyBuffer_Release(), hence the added "goto cleanup" in a failure case. We don't bump the extension version because the API has not changed. Differential Revision: https://phab.mercurial-scm.org/D4840