Sync Sphinx requirement

1. Sync sphinx dependency with global requirements. It caps python 2 since
sphinx 2.0 no longer supports Python 2.7.
2. Update some URLs to latest

Change-Id: Ie9a41cece52c12168b1e73770a90521124f8c6d9

HACKING.rst

@@ -43,7 +43,7 @@ Osbash
 ~~~~~~
 
 Osbash is written in BASH and follows conventions of DevStack:
-`devstack <http://devstack.org/>`_.
+`devstack <https://docs.openstack.org/devstack/latest/>`_.
 
 DevStack bash style guidelines can be found at the bottom of:
 https://opendev.org/openstack/devstack/src/branch/master/HACKING.rst
@@ -107,7 +107,7 @@ Submitting patches
 These documents will help you submit patches to OpenStack projects (including
 this one):
 
-- http://docs.openstack.org/infra/manual/developers.html#development-workflow
+- https://docs.openstack.org/infra/manual/developers.html#development-workflow
 - https://wiki.openstack.org/wiki/GitCommitMessages
 
 If you change the behavior of the scripts as documented in the replace-guides,
@@ -122,7 +122,7 @@ Reviewing
 ---------
 
 Learn how to review (or what to expect when having your patches reviewed) here:
-- http://docs.openstack.org/infra/manual/developers.html#development-workflow
+- https://docs.openstack.org/infra/manual/developers.html#development-workflow
 
 TODO
 ----

doc/requirements.txt

@@ -1,3 +1,8 @@
-sphinx!=1.6.6,!=1.6.7,>=1.6.2 # BSD
+# 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,>=1.6.2;python_version>='3.4'  # BSD
 sphinx-testing # BSD
 openstackdocstheme>=1.19.0 # Apache-2.0