Drop `tests` label if new commits extend the scope outside tests

[johnslavik]

Example cases where tests label remained, but the PR did not finally touch tests only:

• gh-143046: Make asyncio REPL respect the -q flag (quiet mode) cpython#143047 (started with a failing test, then implemented the fix)
• gh-140648: Make asyncio REPL respect the -I flag (isolated mode) cpython#143045 (started with a failing test, then implemented the fix)

If this isn't overly complicated, let's ensure the labels are up-to-date with every new commit.