Update constraints for eventlet to v0.21.0

Eventlet v0.20.0 is affected by [0] causing neutron-rpc-server to fail
with SSL enabled. The issue is fixed in v0.21.0

[0] eventlet/eventlet#371

Closes-Bug: #1884808
Change-Id: I9c8e226166358c7d4e3b5cd57512ac08497aee02
Signed-off-by: Andrii Ostapenko <andrii.ostapenko@att.com>

upper-constraints.txt

@@ -135,7 +135,7 @@ google-auth===1.5.0
 kazoo===2.5.0
 XStatic-roboto-fontface===0.5.0.0
 pyudev===0.21.0
-eventlet===0.20.0
+eventlet===0.21.0
 openstack-doc-tools===1.8.0
 frozendict===1.2
 oslo.messaging===8.1.4