[ussuri][goal] Updates for python 2.7 drop

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

openstack-ansible repos only need updates on requirements
and tox file.

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://governance.openstack.org/tc/goals/selected/ussuri/drop-py27.html

Change-Id: Icd7542d4fd9d2c5636d55ac633e4422db15e48e9

bindep.txt

@@ -29,9 +29,6 @@ gcc
 git-core          [platform:dpkg platform:suse]
 libssl-dev        [platform:dpkg]
 libffi-dev        [platform:dpkg]
-python2.7         [platform:dpkg]
-python-apt        [platform:dpkg]
-python-dev        [platform:dpkg]
 python3           [platform:dpkg]
 python3-apt       [platform:dpkg]
 python3-dev       [platform:dpkg]
@@ -41,8 +38,8 @@ gcc-c++           [platform:rpm]
 git               [platform:rpm !platform:suse]
 libffi-devel      [platform:rpm]
 openssl-devel     [platform:rpm]
-python-devel      [platform:rpm]
-python2-dnf       [platform:fedora]
+python3-devel     [platform:rpm]
+python3-dnf       [platform:fedora]
 
 # Base requirements for Gentoo
 git               [platform:gentoo]

tox.ini

@@ -2,9 +2,11 @@
 minversion = 2.0
 skipsdist = True
 envlist = docs,linters,functional
+ignore_basepython_conflict = True
 
 [testenv]
 usedevelop = True
+basepython = python3
 install_command =
     pip install -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
 commands =
@@ -31,7 +33,6 @@ setenv =
     WORKING_DIR={toxinidir}
 
 [testenv:docs]
-basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands =
     bash -c "rm -rf doc/build"
@@ -39,7 +40,6 @@ commands =
     sphinx-build --keep-going -b html doc/source doc/build/html
 
 [testenv:pdf-docs]
-basepython = python3
 deps = {[testenv:docs]deps}
 whitelist_externals =
   make
@@ -75,17 +75,14 @@ commands =
 ignore=F403
 
 [testenv:bashate]
-basepython = python3
 commands =
     bash -c "{toxinidir}/tests/common/test-bashate.sh"
 
 [testenv:ansible-syntax]
-basepython = python3
 commands =
     bash -c "{toxinidir}/tests/common/test-ansible-syntax.sh"
 
 [testenv:ansible-lint]
-basepython = python3
 commands =
     bash -c "{toxinidir}/tests/common/test-ansible-lint.sh"
 
@@ -94,7 +91,6 @@ commands =
     bash -c "{toxinidir}/tests/common/test-ansible-functional.sh"
 
 [testenv:linters]
-basepython = python3
 commands =
     bash -c "{toxinidir}/tests/common/test-ansible-env-prep.sh"
     {[testenv:pep8]commands}