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.

local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed