Bump lower-constraints for eventlet to 0.26.1

eventlet before 0.32.0 have issues with
dnspython>=2.0.0, eventlet-0.26.1+[1] have
pinned dnspython for this, so let's use that
version instead for lower-constraints.

This should help with timeouts seen in
openstack-tox-lower-constraints job.

[1] https://github.com/eventlet/eventlet/commit/4236143

Change-Id: I09826681cf61eb0ff6c82421e8e85a2044e95a4a

lower-constraints.txt

@@ -19,7 +19,7 @@ doc8==0.8.1
 docutils==0.11
 dogpile.cache==0.6.5
 dulwich==0.15.0
-eventlet==0.25.1
+eventlet==0.26.1
 extras==1.0.0
 fasteners==0.7.0
 fixtures==3.0.0

requirements.txt

@@ -8,7 +8,7 @@ PasteDeploy>=1.5.0 # MIT
 Routes>=2.3.1 # MIT
 debtcollector>=1.19.0 # Apache-2.0
 decorator>=4.1.0 # BSD
-eventlet>=0.25.1 # MIT
+eventlet>=0.26.1 # MIT
 pecan>=1.3.2 # BSD
 httplib2>=0.9.1 # MIT
 requests>=2.18.0 # Apache-2.0