pip: Blacklist eventlet 0.23.0
This is to sync with OpenStack requirements.
# NOTE: OpenStack avoids some versions of eventlet, because of the
# following issue.
# https://github.com/eventlet/eventlet/issues/401
-eventlet!=0.18.3,>=0.18.2,!=0.20.1,!=0.21.0
+eventlet!=0.18.3,>=0.18.2,!=0.20.1,!=0.21.0,!=0.23.0
msgpack>=0.3.0 # RPC library, BGP speaker(net_cntl)
netaddr
oslo.config>=1.15.0
