gnu: Blanket: Add python to native-inputs.

Since commit 5d20d7e1369fc7d93de19c0bd219937d697ceae6, meson no longer
propagates Python.

* gnu/packages/task-management.scm (blanket)[native-inputs]: Add python.

gnu/packages/task-management.scm

@@ -223,7 +223,8 @@ a task.")
        ("glib:bin" ,glib "bin")
        ("gobject-introspection" ,gobject-introspection)
        ("gtk+:bin" ,gtk+ "bin")
-       ("pkg-config" ,pkg-config)))
+       ("pkg-config" ,pkg-config)
+       ("python" ,python)))
     (inputs
      (list appstream-glib
            gsettings-desktop-schemas