Add remaining Python type annotations to build backend

[cthoyt]

Summary

This PR add remaining Python type annotations to build backend, specifically in python/uv/_build_backend.py.

Test Plan

• Running uvx ruff check --select ANN python/ checks that annotations are available everywhere.
  
  Adding this into ruff.toml would cause all of the scripts to get checked, which I could update if requested, but currently is out of scope for this PR.
• Running uvx mypy --strict python/ checks that there are no typing issues

[konstin]

Thanks!

Given that PEP 517 explicitly marks config_settings as escape hatch, with a SHOULD on a string-string multi-dict input, these dicts are either dict[Any, str | list[str]] or dict[Any, Any]. Since we don't read them, i'd go with dict[Any, Any] for simplicity.

[cthoyt]

@konstin the only issue with dict[Any, Any] is that it requires globally importing (part of) the typing module, even if the type annotation itself is hiding in a string.

Given uv is trying to be as performant as possible, this could be an issue. What do you think?

[notatallshaw]

Given uv is trying to be as performant as possible, this could be an issue. What do you think?

If your goal is to avoid importing typing at runtime I beleive this is accepted by all type checkers:

TYPE_CHECKING = False
if TYPE_CHECKING:
    from typing import Any

def foo() -> "dict[Any, Any]": ...

[cthoyt]

Thanks @notatallshaw, I gave that a try and it seemed to work! The results are in 449fa44.