Drop explicit basepython in tox config

We expect tox to be installed with python3

Author: philpep

tox.ini

@@ -10,19 +10,16 @@ usedevelop=True
 passenv=HOME TRAVIS DOCKER_CERT_PATH DOCKER_HOST DOCKER_TLS_VERIFY
 
 [testenv:flake8]
-basepython=python3
 deps=hacking
 commands=flake8 {posargs:testinfra test}
 
 [testenv:pylint]
-basepython=python3
 deps=
     pylint
     -rtest-requirements.txt
 commands=pylint {posargs:testinfra}
 
 [testenv:sphinxdoc]
-basepython=python3
 deps=-rdev-requirements.txt
 commands=sphinx-build -W -b html doc/source doc/build