Fix support of Werkzeug 2.1.0

Since https://github.com/pallets/werkzeug/commit/74601150b559c73a8c8732b4c67bbc6377d1a7df
(published yesterday as part of version 2.1.0), Werkzeug crashes when WERKZEUG_RUN_MAIN
is true but WERKZEUG_SERVER_FD is not provided.

This is legitimate, because applications should not set WERKZEUG_RUN_MAIN.

This commit rewrites the hack hiding the server banner so it still works
with this Werkzeug version.