gnu: python-graphviz: Add native input.

* gnu/packages/graphviz.scm (python-graphviz)[native-inputs]: Add
python-mock.

This partially reverts commit 3d388fe3d0

gnu/packages/graphviz.scm

@@ -194,6 +194,7 @@ interfaces for other technical domains.")
      (list unzip
            ;; For tests.
            graphviz
+           python-mock
            python-pytest
            python-pytest-cov
            python-pytest-mock))