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: If1bbf13b94f301d3d3330439c9352050ee030ed7
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

requirements.txt

@@ -9,7 +9,7 @@ oslo.concurrency>=3.26.0 # Apache-2.0
 stevedore>=1.20.0 # Apache-2.0
 enum34>=1.0.4;python_version=='2.7' or python_version=='2.6' or python_version=='3.3' # BSD
 doc8>=0.6.0 # Apache-2.0
-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
 six>=1.10.0 # MIT
 
 jsonschema<3.0.0,>=2.6.0 # MIT