PyO3 0.21.1

This release is a set of touch-ups to PyO3 0.21 to fix cases found as adoption of PyO3 0.21.0 progressed.

The new PyBackedStr and PyBackedBytes types have had a large range of standard traits implemented, including Clone, PartialEq, Hash, PartialOrd, Display, Debug, Send and Sync.

The argument to a #[setter] function now accepts the full range of input types as any other function argument in a #[pymethods] block, and will now correctly handle a #[pyo3(from_py_with = "...")] annotation.

Some procedural macro output has been adjusted to reuse more code and make it easier to reach complete code coverage in projects using PyO3. The import_exception_bound! macro has been added to further assist with coverage during migration, by skipping generating code which is only needed to use the exception as a GIL Ref.

Finally, the experimental-async feature now correctly handles async fn in #[pymethods] which have a &self or &mut self receiver plus additional arguments.

Thank you to the following users for the improvements:

@alex
@davidhewitt
@geo7
@Icxolu
@reswqa
@rikushoney
@tisonkun