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: I6e8c0d17fb96e507547b61d1d8523b259d1fc204
Closes-Bug:  #1801657

tox.ini

@@ -75,16 +75,19 @@ ignore=F403,H303
 
 
 [testenv:bashate]
+basepython = python3
 commands =
     bash -c "{toxinidir}/tests/common/test-bashate.sh"
 
 
 [testenv:ansible-syntax]
+basepython = python3
 commands =
     bash -c "{toxinidir}/tests/common/test-ansible-syntax.sh"
 
 
 [testenv:ansible-lint]
+basepython = python3
 commands =
     bash -c "{toxinidir}/tests/common/test-ansible-lint.sh"