PyO3 0.16.6

This release is a tactical set of soundness fixes identified for the PyCapsule bindings released in PyO3 0.16. To avoid breaking API changes capsules created with PyCapsule::new and PyCapsule::new_with_destructor will now leak their contents (and not call the destructor) if released on a thread other than the one they were created.

PyO3 0.17 will be released shortly with breaking API changes which resolve the PyCapsule issues with better design (e.g. the destructor has a Send bound added). Users are encouraged to upgrade at their earliest convenience.

Thanks to @saethlin for reporting the issue, and to @adamreichold and @davidhewitt for implementing the resolution.