Revert "pytest.ini: Remove registration of flaky marker"

This reverts commit 0f5b4d58e2.

Seems this is needed when freezing, otherwise we get a "'flaky' not a
registered marker" error despite the plugin being installed.

Conflicts:
	pytest.ini

pytest.ini

@@ -16,6 +16,7 @@ markers =
     pyqt531_or_newer: Needs PyQt 5.3.1 or newer.
     xfail_norun: xfail the test with out running it
     ci: Tests which should only run on CI.
+    flaky: Tests which are flaky and should be rerun
 qt_log_level_fail = WARNING
 qt_log_ignore =
     ^SpellCheck: .*