chore(pkg): use `typing.Self` annotation

[eonu]

Description

Use typing.Self type annotation instead of forward references for self.

Checklist

• [x] I have added new tests (if necessary).
• [x] I have ensured that tests and coverage are passing on CI.
• [x] I have updated any relevant documentation (if necessary).
• [x] I have used a descriptive pull request title.

[coveralls]

Pull Request Test Coverage Report for Build 7158edaf-ce72-4184-b89e-cfa1dea28ef0

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 90.594%

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Totals
Change from base Build 37a99751-b074-4a42-919e-5f105151c593:	0.0%
Covered Lines:	992
Relevant Lines:	1095

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💛 - Coveralls