Incorrect type annotation for iter_lines

The type annotation for Response.iter_lines misses that chunk_size can be None, causing valid code to fail type checks.

This seems pretty trivial to fix (e.g. https://github.com/psf/requests/compare/main...grantisu:requests:gmathews/tiny-type-fix), but I'm not able to open a PR since I'm not a collaborator on the repo.