Do not depend on python mediator.

--- libnotify-0.8.8/tests/meson.build.orig
+++ libnotify-0.8.8/tests/meson.build
@@ -2,7 +2,7 @@
 test_env.set('G_DEBUG', 'fatal-criticals')
 
 dbus_run_session = find_program('dbus-run-session', required: false)
-py = python.find_installation('python3',
+py = python.find_installation(
   modules: ['dbus', 'dbusmock'],
   required: false)
 xvfb_run = find_program('xvfb-run', required: false)