Split tool.stubtest.platforms metadata key (#13746)

Co-authored-by: Avasam <samuel.06@hotmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

stubs/keyboard/METADATA.toml

@@ -6,4 +6,4 @@ upstream_repository = "https://github.com/boppreh/keyboard"
 # It's only by possible mouse buttons and event literal types.
 # As well as returning a tuple of int/long from keyboard.mouse.get_position
 # The "mouse" module is obsoleted by the "mouse" package.
-# platforms =
+# ci_platforms = ["linux"]