Add type annotations / type checking

[RagnarGrootKoerkamp]

This may improve long term maintainability and give better errors during development.

Roadmap (last update 2025-02-16)

• #102 Part 1: Initial setup for type checking #392
• #385 Add limits #415 / Stricter parsing of problem.yaml #419
• Mark constants in config as Final #422
• #102 More types in util.py #423
• TODO in .pre-commit-config.yaml: Enable --strict flag for mypy once everything has type annotations
  • Related, follow-up for #102 More types in util.py #423 (comment): enforce type annotations for class fields
• TODO in pyproject.toml: Widdle out star-imports
• Add typed namespace for config.args
• Add types for testdata.yaml
• Linter rules to avoid deprecated symbols (e.g. typing.Sequence → collections.abc.Sequence: Add validation of generically generated invalid files #420 (comment))?
• Compare mypy with pyright