feat: reduce unnessessarry type casting of set/SortedSet when useing setter

[jkowalleck]

show what would happen, if the unnecessary type cast of set/SortedSet was omitted.

unnecessary on runtime (as all unit/functional/integration tests are passing),
but mypy is not able to detect that the setter has a wider type definition than the getter...
see the issue python/mypy#3004
and the decision that the issue is a feature-lack/bug: python/mypy#3004 (comment)

this PR is intended to last for a while and be rebased, whenever needed,
to test if mypy implemented the missing feature, some day.

[RodneyRichardson]

Is this obsolete now we're using SortedSet?

[jkowalleck]

Good hint.
Will rebase on master and see what happens. Maybe a breakthrough, maybe another hanging demo-PR.

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update:
did the rebase, not surpised that it is still relevant and still open.
the issue with mypi is still there. since the experts in the linked mypy issue report guide to use tyoe: ignore i did this, so we could have this PR reviewed & fixed