remove py27 in tox.ini from tripleo-heat-templates

Remove python 2 from envlist parameter. Check the link:

https://etherpad.openstack.org/p/drop-python2-support
https://governance.openstack.org/tc/goals/selected/ussuri/drop-py27.html

The plan is drop the python 2 support from OpenStack in Ussuri release.
Move the basepython parameter.

Change-Id: I794fe6677328d74bef305dcfc15ab81038ae2b87

tox.ini

@@ -1,9 +1,10 @@
 [tox]
 minversion = 2.0
 skipsdist = True
-envlist = pep8,py27,py37,tht
+envlist = pep8,py37,tht
 
 [testenv]
+basepython = python3
 usedevelop = True
 deps =
        -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
@@ -12,11 +13,9 @@ deps =
 commands = python setup.py testr --slowest --testr-args='{posargs}'
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:pep8]
-basepython = python3
 whitelist_externals =
     bash
 commands =
@@ -28,7 +27,6 @@ commands =
     flake8 --exclude releasenotes --ignore {[testenv:flake8]ignore}
 
 [testenv:flake8]
-basepython = python3
 # E125 is deliberately excluded. See
 # https://github.com/jcrocholl/pep8/issues/126. It's just wrong.
 #
@@ -56,45 +54,37 @@ ignore = E121,E122,E123,E124,E125,E126,E127,E128,E129,E131,E251,H405,W503,W504,E
 exclude = releasenotes
 
 [testenv:templates]
-basepython = python3
 commands = python ./tools/process-templates.py
 
 [testenv:clean]
-basepython = python3
 commands = python ./tools/process-templates.py -c
 
 [testenv:releasenotes]
-basepython = python3
 whitelist_externals =
     bash
 commands = bash -c tools/releasenotes_tox.sh
 
 [testenv:cover]
-basepython = python3
 commands = python setup.py test --coverage --coverage-package-name=tripleo_heat_templates --testr-args='{posargs}'
 
 [testenv:genconfig]
-basepython = python3
 commands =
            python ./tools/process-templates.py
            python ./tripleo_heat_templates/environment_generator.py sample-env-generator/
 
 [testenv:genroledata]
-basepython = python3
 whitelist_externals =
     bash
 commands =
     bash -c tools/roles-data-generate-samples.sh
 
 [testenv:lower-constraints]
-basepython = python3
 deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt
   -r{toxinidir}/requirements.txt
 
 [testenv:tht]
-basepython = python3
 usedevelop = True
 setenv =
     ANSIBLE_FORCE_COLOR=1