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: I8fd0c040a8c827d2f3b6f0ae796b1459713d3bf2

tox.ini

@@ -4,7 +4,6 @@ minversion = 2.3
 skipsdist = True
 
 [testenv]
-basepython = python3
 setenv = VIRTUAL_ENV={envdir}
          OS_STDOUT_CAPTURE=1
          OS_STDERR_CAPTURE=1
@@ -15,18 +14,23 @@ commands = find {toxinidir} -type f -not -path '{toxinidir}/.tox/*' -not -path '
 deps = -r{toxinidir}/test-requirements.txt
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:flake8]
+basepython = python3
 description = Dummy environment to allow flake8 to be run in subdir tox
 
 [testenv:pep8]
+basepython = python3
 description = Dummy environment to allow pep8 to be run in subdir tox
 
 [testenv:pylint]
+basepython = python3
 description = Dummy environment to allow pylint to be run in subdir tox
 
 [testenv:linters]
+basepython = python3
 whitelist_externals = bash
 commands =
   bash -c "find {toxinidir}        \
@@ -54,6 +58,7 @@ whitelist_externals =
     reno
 
 [testenv:newnote]
+basepython = python3
 # Re-use the releasenotes venv
 envdir = {toxworkdir}/releasenotes
 deps = -r{toxinidir}/doc/requirements.txt