Add basepython=python3.5 to [testenv] in tox.ini

Running tox -e lint in an environment with python2.7 for example
as the default python interpreter causes the following errors
to be emitted:

lint runtests: commands[0] | yapf -rd /opt/stack/ucp/promenade/promenade /opt/stack/ucp/promenade/tests
lint runtests: commands[1] | flake8 /opt/stack/ucp/promenade/promenade
/opt/stack/ucp/promenade/promenade/renderer.py:18:39: E999 SyntaxError: invalid syntax
/opt/stack/ucp/promenade/promenade/config.py:17:19: E999 SyntaxError: invalid syntax
/opt/stack/ucp/promenade/promenade/tar_bundler.py:17:20: E999 SyntaxError: invalid syntax
/opt/stack/ucp/promenade/promenade/logging.py:76:12: E999 SyntaxError: invalid syntax
/opt/stack/ucp/promenade/promenade/generator.py:11:33: E999 SyntaxError: invalid syntax
/opt/stack/ucp/promenade/promenade/builder.py:15:33: E999 SyntaxError: invalid syntax
/opt/stack/ucp/promenade/promenade/pki.py:59:43: E999 SyntaxError: invalid syntax
/opt/stack/ucp/promenade/promenade/cli.py:13:16: E999 SyntaxError: invalid syntax

This PS just adds basepython=python3.5 to the base [testenv] category
from which jobs like tox -e lint inherit causing tox to use the python3.5
interpreter. Since Promenade only supports Python 3.5+ (since it isn't
tested against anything else), this won't break anything.

Change-Id: I549a02dbfa5f3b53bab26b87ad3042192e315530

tox.ini

@@ -1,6 +1,9 @@
 [tox]
 envlist = lint,unit,bandit,docs
 
+[testenv]
+basepython=python3.5
+
 [testenv:unit]
 setenv =
   PYTHONWARNING=all
@@ -29,7 +32,6 @@ commands =
     yapf -ir {toxinidir}/promenade {toxinidir}/tests
 
 [testenv:freeze]
-basepython=python3.5
 deps = -r{toxinidir}/requirements-direct.txt
 recreate = True
 whitelist_externals = sh