[3.13] gh-57141: Make shallow argument to filecmp.dircmp keyword-only (GH-121767) (#121777)

It is our general practice to make new optional parameters keyword-only,
even if the existing parameters are all positional-or-keyword. Passing
this parameter as positional would look confusing and could be error-prone
if additional parameters are added in the future.
(cherry picked from commit 50eec501fef46f0887df6f2f47d74f4defb35515)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>