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: I6a4531e148cf2695bbf5fea34ee8b539942bbc39
Closes-Bug:  #1801657

tox.ini

@@ -24,11 +24,13 @@ commands = python -m pre_commit run --source HEAD^ --origin HEAD
 
 # deprecated: use linters instead. kept only as a convenience alias
 [testenv:pep8]
+basepython = python3
 envdir = {toxworkdir}/linters
 whitelist_externals = {[testenv:linters]whitelist_externals}
 commands = {[testenv:linters]commands}
 
 [testenv:cireport]
+basepython = python3
 passenv =
   SSH_AUTH_SOCK
 commands = python scripts/tripleo-jobs-gerrit.py {posargs}
@@ -36,6 +38,7 @@ commands = python scripts/tripleo-jobs-gerrit.py {posargs}
 # This environment can be used to quickly validate that all needed system
 # packages required to successfully execute test targets are installed
 [testenv:bindep]
+basepython = python3
 # Do not install any requirements. We want this to be fast and work even if
 # system dependencies are missing, since it's used to tell you what system
 # dependencies are missing! This also means that bindep must be installed
@@ -50,6 +53,7 @@ changedir = scripts/emit_releases_file
 commands = pytest {posargs:--cov=emit_releases_file}
 
 [testenv:py35]
+basepython = python3
 deps={[testenv]deps}
 changedir = {[testenv:py27]changedir}
 commands = {[testenv:py27]commands}