Allow some custom return types to be annotated as generic classes at runtime

[IvanIsCoding]

Related to #1345 Follow up of #1348

True diff: https://github.com/IvanIsCoding/rustworkx/compare/class-get-item...IvanIsCoding:rustworkx:pep563-custom-return-types

After #1348 is merged this should be easier to review

[coveralls]

Pull Request Test Coverage Report for Build 12818984732

Details

• 26 of 26 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.006%) to 95.817%

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Totals
Change from base Build 12817417491:	0.006%
Covered Lines:	18369
Relevant Lines:	19171

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💛 - Coveralls

[IvanIsCoding]

Putting this on-hold again as it could profit from the multiple-pymethods feature from PyO3