uncap eventlet

We will manage the eventlet version using constraints now. See the
thread starting at
http://lists.openstack.org/pipermail/openstack-dev/2018-April/129096.html
for more details.

Change-Id: Ibfb9e78a8eb734f7246f0d06c3e7e03c72219cb1
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

requirements.txt

@@ -4,7 +4,7 @@
 
 pbr!=2.1.0,>=2.0.0 # Apache-2.0
 Babel!=2.4.0,>=2.3.4 # BSD
-eventlet!=0.18.3,!=0.20.1,<0.21.0,>=0.18.2 # MIT
+eventlet!=0.18.3,!=0.20.1,>=0.18.2 # MIT
 oslo.concurrency>=3.26.0 # Apache-2.0
 oslo.log>=3.36.0 # Apache-2.0
 oslo.i18n>=3.15.3 # Apache-2.0