[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.

Add bindep.txt file so that wheels can be build.

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

Change-Id: I51c3bcf161b363e4b188175b75097d19a0d10a74

.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

bindep.txt

@@ -0,0 +1,10 @@
+# This is a cross-platform list tracking distribution packages needed for install and tests;                                                            # see http://docs.openstack.org/infra/bindep/ for additional information.
+
+gcc [platform:rpm test]
+libffi-dev [platform:dpkg]
+libffi-devel [platform:redhat]
+libffi48-devel [platform:suse]
+virtual/libffi [platform:gentoo]
+libpcre3-dev [platform:dpkg test]
+pcre-devel [platform:rpm test]
+

tox.ini

@@ -1,9 +1,11 @@
 [tox]
-minversion = 1.6
+minversion = 3.1.1
-envlist = docs,py27
+envlist = docs,pep8
 skipsdist = True
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 setenv = VIRTUAL_ENV={envdir}
 install_command = pip install -U {opts} {packages}
@@ -11,11 +13,9 @@ deps = -r{toxinidir}/requirements.txt
 commands = stestr run --slowest {posargs}
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:docs]
-basepython = python3
 whitelist_externals =
   rm
 commands =