Validataclass 0.12 and mypy

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DRAFT: This PR is based on #5 which needs to be merged first!

DRAFT: Also, the PR uses a dev version of validataclass 0.12.0 because there's no official release yet. Before merging, the dependencies need to be adjusted accordingly!

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This PR updates the library to use validataclass 0.12.0 (which as of now is not released yet), enables mypy, and fixes a lot of typing issues. It's basically a big typing cleanup.

The PR also adds support for Python 3.13 and 3.14 (since those are supported with validataclass 0.12.0).

Since validataclass 0.12.0 is not released yet, I had to build a dev version of it, so that I could already work on making it compatible. Of course, the validataclass version needs to be updated before merging this.

Full list of changes:

• WIP: Update to dev version of validataclass 0.12.0
• Compatibility changes for validataclass 0.12.0 (typing related and other)
• Add support for Python 3.13 and 3.14
• Use absolute imports (unless same package)
• Don't export type vars to discourage reusing them
• Add mypy to toolchain and to CI, enable strict mode and extra rules
• Refactor search_query_dataclass decorator for better typing
• Fix incorrect short-circuiting of substring search filters
  • (I have no idea if this ever worked in the first place, it doesn't look like it could have worked 😅 )
• Fix a lot of typing issues
• Change max line length in flake8 config from 140 to 120 (and reformat some code)
• Add py.typed file to make the package PEP 561 compatible