[ussuri][goal] Drop python 2.7 support

OpenStack is dropping the py2.7 support in ussuri cycle.

specs repo either has py27 job or requirement or tox env.

Doc building still use deprcated oslosphinx and incompatible
version of yasfb which lead to error-

sphinx.errors.ExtensionError: Could not import extension yasfb (exception: cannot import name 'logging')

This commit replace oslosphinx with openstackdocstheme ti fix the error.

Ussuri Communtiy-wide goal:
https://governance.openstack.org/tc/goals/selected/ussuri/drop-py27.html

Change-Id: I76a9e0e3259ec52564f31a239079cd91da897ee9

.zuul.yaml

@@ -3,7 +3,7 @@
       - openstack-specs-jobs
     check:
       jobs:
-        - openstack-tox-py27
+        - openstack-tox-py37
     gate:
       jobs:
-        - openstack-tox-py27
+        - openstack-tox-py37

doc/source/conf.py

@@ -24,7 +24,7 @@ sys.path.insert(0, os.path.abspath('../..'))
 extensions = [
     'sphinx.ext.autodoc',
     #'sphinx.ext.intersphinx',
-    'oslosphinx',
+    'openstackdocstheme',
     'yasfb',
 ]
 
@@ -75,6 +75,7 @@ pygments_style = 'sphinx'
 # html_theme_path = ["."]
 # html_theme = '_theme'
 # html_static_path = ['static']
+html_theme = "openstackdocs"
 
 # Output file base name for HTML help builder.
 htmlhelp_basename = '%sdoc' % project

requirements.txt

@@ -1,4 +1,4 @@
 pbr>=0.6,<1.0
-oslosphinx
+openstackdocstheme
-sphinx>=1.1.2,<1.2
+sphinx>=2.0.0
-yasfb>=0.5.1
+yasfb>=0.8.0

tox.ini

@@ -1,9 +1,10 @@
 [tox]
 minversion = 1.6
-envlist = docs,py27
+envlist = docs,pep8
 skipsdist = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 install_command = pip install -U {opts} {packages}
 setenv =
@@ -16,15 +17,12 @@ commands =
   python setup.py testr --slowest --testr-args='{posargs}'
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:docs]
-basepython = python3
 commands = python setup.py build_sphinx
 
 [testenv:spelling]
-basepython = python3
 deps =
    -r{toxinidir}/requirements.txt
    sphinxcontrib-spelling