fix tox python3 overrides

We want to default to running all tox environments under python 3, so
set the basepython value in each environment.

We do not want to specify a minor version number, because we do not
want to have to update the file every time we upgrade python.

We do not want to set the override once in testenv, because that
breaks the more specific versions used in default environments like
py35 and py36.

Change-Id: Ibeb7d21ba2ba3299b206e90753467a7dd0b3f2d1

tox.ini

@@ -23,6 +23,7 @@ commands =
     coverage report --fail-under 75
 
 [testenv:pep8]
+basepython = python3
 # TODO: #deps = hacking==0.7
 deps = flake8
 usedevelop = False
@@ -30,6 +31,7 @@ commands =
     flake8 {posargs:.}
 
 [testenv:pep8-py34]
+basepython = python3
 # TODO: #deps = hacking==0.7
 deps = flake8
 usedevelop = False
@@ -37,6 +39,7 @@ commands =
     flake8 {posargs:.}
 
 [testenv:pep8-py35]
+basepython = python3
 # TODO: #deps = hacking==0.7
 deps = flake8
 usedevelop = False
@@ -51,6 +54,7 @@ show-source = True
 count = True
 
 [testenv:pylint]
+basepython = python3
 deps=
     {[testenv]deps}
     pylint
@@ -84,6 +88,7 @@ commands=pylint fuel_tests
 
 
 [testenv:docs]
+basepython = python3
 changedir = doc
 deps =
     {[testenv]deps}
@@ -91,6 +96,7 @@ deps =
 commands = sphinx-build -b html -d _build/doctrees . _build/html
 
 [testenv:cover]
+basepython = python3
 deps =
     {[testenv]deps}
 commands =