gnu: python-mkdocs: Fix build on python-3.12.

* gnu/packages/python-xyz.scm (python-mkdocs)[native-inputs]: Add
python-setuptools.

Change-Id: Idafcee25dd899cbe8c52479d9270cf287582b4f0
Reviewed-by: Hugo Buddelmeijer <hugo@buddelmeijer.nl>
Signed-off-by: Sharlatan Hellseher <sharlatanus@gmail.com>

gnu/packages/python-xyz.scm

@@ -3498,7 +3498,8 @@ that best match text queries.")
       #:test-flags
       #~(list "discover" "-p" "*tests.py" "mkdocs" "--top-level-directory" ".")))
     (native-inputs
-     (list python-hatchling))
+     (list python-hatchling
+           python-setuptools))
     (propagated-inputs
      (list python-click
            python-ghp-import