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: Id83cb3cdd62517045c45388f88cb3de0e3d75da1

tox.ini

@@ -8,6 +8,7 @@ usedevelop = False
 install_command = pip install {opts} {packages}
 
 [testenv:bashate]
+basepython = python3
 # if you want to test out some changes you have made to bashate
 # against devstack, just set BASHATE_INSTALL_PATH=/path/... to your
 # modified bashate tree
@@ -34,6 +35,7 @@ commands = bash -c "find {toxinidir}             \
          -print0 | xargs -0 bashate -v -iE006 -eE005,E042"
 
 [testenv:docs]
+basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 whitelist_externals = bash
 setenv =
@@ -42,5 +44,6 @@ commands =
   python setup.py build_sphinx
 
 [testenv:venv]
+basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands = {posargs}