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: I6259f258917217f4d2ce8a1b5cbe35bd06ac58d4
Closes-Bug:  #1802299

tox.ini

@@ -21,17 +21,21 @@ commands =
 sitepackages = False
 
 [testenv:pep8]
+basepython = python3
 sitepackages = False
 commands =
   flake8 {posargs}
 
 [testenv:debug]
+basepython = python3
 commands = oslo_debug_helper {posargs}
 
 [testenv:genconfig]
+basepython = python3
 commands = oslo-config-generator --config-file=etc/gceapi/gceapi-config-generator.conf
 
 [testenv:cover]
+basepython = python3
 # Also do not run test_coverage_ext tests while gathering coverage as those
 # tests conflict with coverage.
 commands =
@@ -39,6 +43,7 @@ commands =
     --testr-args='^(?!.*test.*coverage).*$'
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [flake8]