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.

  $ LANG=nonexistent LC_ALL=nonexistent LANGUAGE=nonexistent hg version -q
  Mercurial Distributed SCM (version *) (glob)