wireprotoserver: ensure that output stream gets flushed on exception Previously flush was happening due to Python finalizer being run on `BufferedWriter`. With upgrade to Python 3.11 this started randomly failing. My guess is that the finalizer on the raw `FileIO` object may be running before the finalizer of `BufferedWriter` has a chance to run. At any rate, since we're not relying on finalizers in the happy case we should also not rely on them in case of exception.

#require docutils gettext

  $ $TESTDIR/check-gendoc sv
  checking for parse errors