Fix lower-constraints gate after switch to py36

To make lower-constraints gate work with Python 3 we need to upgrade
eventlet to 0.22.0. Also it's now safe to drop Python 2 entry for
Sphinx.

Change-Id: I50663c428cf5524526055374e53964b86609f653
Depends-On: https://review.opendev.org/#/c/693631/

doc/requirements.txt

@@ -1,7 +1,6 @@
 # The order of packages is significant, because pip processes them in the order
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
-sphinx!=1.6.6,!=1.6.7,>=1.6.2,<2.0.0;python_version=='2.7' # BSD
 sphinx!=1.6.6,!=1.6.7,!=2.1.0,>=1.6.2;python_version>='3.4' # BSD
 openstackdocstheme>=1.20.0 # Apache-2.0
 reno>=2.5.0 # Apache-2.0

lower-constraints.txt

@@ -21,7 +21,7 @@ docutils==0.11
 dogpile.cache==0.6.5
 dulwich==0.19.0
 enum-compat==0.0.2
-eventlet==0.18.2
+eventlet==0.22.0
 extras==1.0.0
 fasteners==0.14.1
 fixtures==3.0.0

requirements.txt

@@ -7,7 +7,7 @@ Flask!=0.11,>=0.12.3 # BSD
 kuryr-lib>=0.5.0 # Apache-2.0
 pbr!=2.1.0,>=2.0.0 # Apache-2.0
 requests>=2.14.2 # Apache-2.0
-eventlet!=0.18.3,!=0.20.1,!=0.21.0,>=0.18.2 # MIT
+eventlet>=0.22.0 # MIT
 netaddr>=0.7.19 # BSD
 openstacksdk>=0.13.0  # Apache-2.0
 oslo.cache>=1.26.0 # Apache-2.0