Drop python 2.7 support and testing

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

ironic-python-agent-builder is ready with python 3 and ok to drop
the python 2.7 support.

Complete discussion & schedule can be found in
- http://lists.openstack.org/pipermail/openstack-discuss/2019-October/010142.html
- https://etherpad.openstack.org/p/drop-python2-support

Ussuri Communtiy-wide goal - https://review.opendev.org/#/c/691178/

Change-Id: I4dc5fe4cbb2ec4e604e18f77da4b5723ab7f9c2b

releasenotes/notes/drop-python2-534124afa50f62dd.yaml

@@ -0,0 +1,7 @@
+---
+upgrade:
+  - |
+    Python 2.7 support has been dropped. Last release of
+    ironic-python-agent-builder to support Python 2.7 is OpenStack Train. The
+    minimum version of Python now supported by ironic-python-agent-builder is
+    Python 3.6.

test-requirements.txt

@@ -7,7 +7,6 @@ hacking>=1.0.0,<1.2.0 # Apache-2.0
 flake8-import-order>=0.13 # LGPLv3
 # documentation
 doc8>=0.6.0 # Apache-2.0
-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,!=2.1.0,>=1.6.2;python_version>='3.4' # BSD
 openstackdocstheme>=1.20.0 # Apache-2.0
 # releasenotes

tox.ini

@@ -6,33 +6,29 @@ skipsdist = True
 [testenv]
 usedevelop = True
 install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
+basepython = python3
 setenv =
    VIRTUAL_ENV={envdir}
    PYTHONWARNINGS=default::DeprecationWarning
 deps = -r{toxinidir}/test-requirements.txt
 
 [testenv:pep8]
-basepython = python3
 commands =
     flake8 ironic_python_agent_builder
     doc8 doc/source README.rst CONTRIBUTING.rst
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:docs]
-basepython = python3
 commands = python setup.py build_sphinx
 
 [testenv:pdf-docs]
 whitelist_externals = make
-basepython = python3
 commands = sphinx-build -b latex doc/source doc/build/pdf
            make -C doc/build/pdf
 
 [testenv:releasenotes]
-basepython = python3
 commands =
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html