tests: Add Tox config for Python 2.6 and 2.7

I could not get Python 2.5 or earlier to work. Too many packages
(critically docker) don't support it.

dev_requirements.txt

 -r docs/docs-requirements.txt
 ansible==2.3.1.0
-Django==1.11.5  # for module_finder_test
-docker==2.5.1
-docker[tls]==2.5.1
+Django==1.6.11; python_version < '2.7'
+Django==1.11.5; python_version >= '2.7' # for module_finder_test
+https://github.com/docker/docker-py/archive/1.10.6.tar.gz; python_version < '2.7'
+docker[tls]==2.5.1; python_version >= '2.7'
 mock==2.0.0
 pytest-catchlog==1.2.2
 pytest==3.1.2
+PyYAML==3.11; python_version < '2.7'
+PyYAML==3.12; python_version >= '2.7'
 unittest2==1.1.0
+# Fix InsecurePlatformWarning while creating py26 tox environment
+# https://urllib3.readthedocs.io/en/latest/advanced-usage.html#ssl-warnings
+urllib3[secure]; python_version < '2.7.9'

docs/docs-requirements.txt

 Sphinx==1.7.1
-sphinx-autobuild==0.7.1
+sphinx-autobuild==0.6.0 # Last version to support Python 2.6
 sphinxcontrib-programoutput==0.11

tox.ini

+[tox]
+envlist =
+    py26,
+    py27,
+
+[testenv]
+deps =
+    -r{toxinidir}/dev_requirements.txt
+
+commands =
+    {posargs:./test.sh}
+
+[testenv:docs]
+basepython = python
+changedir = docs
+commands =
+    sphinx-build -W -b html -d {envtmpdir}/doctrees . {envtmpdir}/html