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. Remove unncessary "=="

Change-Id: I2c87490faba283c4d4bcb3fbe386955a49601945

releasenotes/source/mitaka.rst

@@ -1,6 +1,6 @@
-===================================
- Mitaka Series Release Notes
-===================================
+===========================
+Mitaka Series Release Notes
+===========================
 
 .. release-notes::
    :branch: origin/stable/mitaka

releasenotes/source/queens.rst

@@ -1,6 +1,6 @@
-===================================
- Queens Series Release Notes
-===================================
+===========================
+Queens Series Release Notes
+===========================
 
 .. release-notes::
    :branch: stable/queens

releasenotes/source/stein.rst

@@ -1,6 +1,6 @@
-===================================
- Stein Series Release Notes
-===================================
+==========================
+Stein Series Release Notes
+==========================
 
 .. release-notes::
    :branch: stable/stein

test-requirements.txt

@@ -6,7 +6,8 @@ hacking>=1.1.0,<1.2.0 # Apache-2.0
 coverage!=4.4,>=4.0 # Apache-2.0
 ddt>=1.0.1 # MIT
 reno>=2.5.0 # Apache-2.0
-sphinx!=1.6.6,!=1.6.7,>=1.6.2 # BSD
+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
 openstackdocstheme>=1.18.1 # Apache-2.0
 oslotest>=3.2.0 # Apache-2.0
 testscenarios>=0.4 # Apache-2.0/BSD