Spec-classes

This is a minor release that improves robustness of skipping attribute default assignment when the class value is a descriptor during initial instantiation (a previously overlooked code pathway when similar changes were made in the previous release).

This a minor release that improves to minor things:

* `functools.cached_property` is now correctly detected as a descriptor and skipped during attribute default preparation.
* In `type_label`: `UnionType`s from Python 3.10+ are show the nested types, and both `Union` and `UnionType` types are now formatted in the Python 3.10 style: e.g. "int | str".

Improves the robustness of sentinel checks.

DO NOT USE. Introduced some sentinel related bugs.

This is a minor release with one minor new feature: support for passing `spec_classes.UNCHANGED` to scalar attribute transformations to skip the setting of new values; especially useful in combination with attribute preparers (where you may not want to commit a null cached value).

This is a minor release with one fix:

* In multithreaded contexts, it is possible to experience issues bootstrapping spec-classes due to race conditions when replacing the __new__ method.