(Temporarily) readd bare support for py27

Revert part of the changes from
Ie1a0cbd82a617dbcc15729647218ac3e9cd0e5a9

This way it is possible again for nova to be installed
using python 2, thus allowing other devstack-based
jobs to work until everything switches to python 3
according the plans.

While py2 support may break anytime, its days are limited anyway.

Change-Id: I99ff84e6c1c033c28d499b0b8ac17ac5b0565f61

requirements.txt

@@ -23,6 +23,7 @@ netaddr>=0.7.18 # BSD
 netifaces>=0.10.4 # MIT
 paramiko>=2.0.0 # LGPLv2.1+
 Babel!=2.4.0,>=2.3.4 # BSD
+enum34>=1.0.4;python_version=='2.7' or python_version=='2.6' or python_version=='3.3' # BSD
 iso8601>=0.1.11 # MIT
 jsonschema>=2.6.0 # MIT
 python-cinderclient!=4.0.0,>=3.3.0 # Apache-2.0

setup.cfg

@@ -6,7 +6,6 @@ description-file =
 author = OpenStack
 author-email = openstack-discuss@lists.openstack.org
 home-page = https://docs.openstack.org/nova/latest/
-python-requires = >=3.6
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology

test-requirements.txt

@@ -3,6 +3,7 @@
 # process, which may cause wedges in the gate later.
 
 hacking>=1.1.0,<1.2.0 # Apache-2.0
+contextlib2>=0.5.5;python_version<'3.0'  # PSF License
 coverage!=4.4,>=4.0 # Apache-2.0
 ddt>=1.0.1 # MIT
 fixtures>=3.0.0 # Apache-2.0/BSD