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: I360cea415303e794c6cbacc26e91f4c8735373fb
Partial-Bug:  #1802032
Signed-off-by:  Andriy Shevchenko <18552437190@163.com>

tox.ini

@@ -4,7 +4,6 @@ minversion = 2.3
 skipsdist = True
 
 [testenv]
-basepython = python3
 install_command = pip install -U {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
          OS_STDOUT_CAPTURE=1
@@ -18,6 +17,7 @@ deps = -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -a -E -W -d doc/build/doctrees -b html doc/source doc/build/html
 
 [testenv:linters]
+basepython = python3
 whitelist_externals = bash
 commands =
   bash -c "find {toxinidir}                   \
@@ -27,4 +27,5 @@ commands =
   doc8 specs/
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}