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: I018f0eec71bd0e062074081dd411c990a33a9ac3
Signed-off-by: Doug Hellmann <doug@doughellmann.com>
diff --git a/requirements.txt b/requirements.txt
index 63cb933..2ecce4e 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -15,4 +15,4 @@
 ddt>=1.0.1 # MIT
 testtools>=2.2.0 # MIT
 testscenarios>=0.4 # Apache-2.0/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