PyO3 0.12.0

This release includes a few careful revisions to the PyO3 API with the intention to make it easier to learn and use. The PyErr type is reworked to implement std::error::Error. The FromPy trait is removed. Finally, the PyObject struct is now just a type alias to Py<PyAny>. While we do not take pleasure in making breaking changes for users' code, we have done so with careful thought to make migration as simple as possible while also allowing the PyO3 API to mature. Please see the migration guide for help upgrading if any of these changes affect you.

Also added is a new #[derive(FromPyObject)] macro, which enables a convenient way to accept arguments of the Python "type" Union. (See the guide entry on this new feature.)

There have been many other improvements and bugfixes too numerous to go into detail here. For the full list, see the CHANGELOG.

Thank you to everyone who contributed code, documentation fixes, design ideas, bug reports, and feedback on the way to PyO3 0.12. The full list of users who commited at least one contribution to PyO3 0.12 is below:

@alex
@birkenfeld
@cathay4t
@davidhewitt
@Hakuyume
@Hywan
@kngwyu
@konstin
@marioortizmanero
@MoritzLangenstein
@mtreinish
@mvaled
@nagisa
@noam93k
@Progdrasil
@programmerjake
@rob-thatcher
@sebpuetz
@vorner
@vthriller