fix compatbility with Python <= 3.9

Further improve type-checking integration for pylance.

Skip calling constructors for non-spec-classes that inherit the spec-…

…classes metadata from a parent class.

fix: Changes to class during deepcopy causes errors.

This could be due to threading or due to non-local mutations during the copying process.

KeyedList: Implement O(1) "contains" checks.

Leverage __tracebackhide__ to reduce clutter in IPython sessions.

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.