Gradually add type hints to submodules,
based on works done in these PRs:
#1262
#1263
#1264

• Start with small functions and functions that require few external dependencies

• Moving gradually one function by one function, in the four test suites: starting from "core", then "compact", "extra", and "noauto"

• Immediate goal for this session - 100% type hints for functions being called in "core" test suite.

• Ultimate goal (outside of this session) is to make the package a typed package, while keeping maintainability.

• Start small in the area with high confidence, then gradually grow one submodule at a time.

• Use mypy as main assistant.

  • mypy is already in "dev" extra dependencies in pyproject.toml
  • mypy configuration is in pyproject.toml

• Use pyright, pyrefly, and pytype for second opinions.

• Start with highly tested submodules first.

• Use native type in type hint (list, tuple, .. instead of List, Tuple, ..)

• Do not use A | B style Union, use full Union[] instead.

• We want to keep compatibility with Python 3.9 strict and allow type inspection tools to work correctly.

• When insert typing imports, put it in appropriate location and order

• Use "if TYPE_CHECKING import" block when possible.

• When reordering the imports, beware of circular import. Read comments near imports to get more information.

• Remove unused imports

• Try to fix the type, the scope logic, or do the casting, before putting #type: ignore comment.

• When dealing with types from external libraries:

  • Check if type information is provided (maybe through stubs). If it is, cast wrapper may not necessary.
  • If return type is not provided, may be it is appropriate to use # type: ignore[no-any-return] annotation for clarity.

• Recheck necessity when do casting.

• Provide stubs as necessary.

• After changes for type hints, recheck for functionality correctness.

• Recheck code, docstring, documentation consistency.

• Recheck formatting with ruff