Add basepython=python3.5 to tox.ini to avoid error in some envs

This PS adds basepython=python3.5 to avoid errors that pop up in
environments where python2.7 may be installed as well, e.g.:

pep8 runtests: commands[0] | flake8
./shipyard_client/cli/action.py:34:25: E999 SyntaxError: invalid syntax
./shipyard_client/api_client/base_client.py:28:27: E999 SyntaxError: invalid syntax
./shipyard_airflow/plugins/deckhand_operator.py:122:25: E225 missing whitespace around operator
./shipyard_airflow/control/ucp_logging.py:100:28: E999 SyntaxError: invalid syntax

Change-Id: I0cb28110d7cb3dcdff93c6010adb5c009a90bb34

tox.ini

@@ -6,6 +6,7 @@ deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 setenv=
   PYTHONWARNING=all
+basepython=python3.5
 commands=
   pytest \
     {posargs}