ZEO: v↑ ZEO5 (5.2.3 -> 5.4.0)

ZEO 5.4.0 fixes several concurrency bugs and provides other
improvements. See changelog in https://pypi.org/project/ZEO/5.4.0/
for details.

component/ZEO/buildout.cfg

@@ -65,7 +65,7 @@ egg     = ZEO
 eggs    = ${:egg}
 
 egg-versions =
-  ZEO = 5.2.3
+  ZEO = 5.4.0
   trollius = 2.2.1
   futures = 3.3.0