Attr: Improve robustness of skipping descriptors during instantiation.

Fix `NoneType` reference in Python <3.10.

fix: Always compare sentinels to object ids to prevent issues with (e…

….g.) pandas.

Increase robustness of sentinel equality checks.

Add support for explicitly indicating that a value should remain unch…

…anged via `UNCHANGED`.

Fix threading issues when multiple threads attempt to bootstrap a spe…

…c-class.

Alias: deepcopy fallback before returning.

Extend `Alias` with support for nested structure.

Extend type checking to `tuple` and `type` annotations.

Fix instance state imputation when spec-classes are coming from deser…

…ialized representations.