Allow non-empty ZEO_MSGPACK environment variable to trigger using msgpack

src/ZEO/runzeo.py

@@ -343,7 +343,7 @@ def create_server(storages, options):
         storages,
         read_only = options.read_only,
         client_conflict_resolution=options.client_conflict_resolution,
-        msgpack=options.msgpack,
+        msgpack=options.msgpack or os.environ.get('ZEO_MSGPACK'),
         invalidation_queue_size = options.invalidation_queue_size,
         invalidation_age = options.invalidation_age,
         transaction_timeout = options.transaction_timeout,