tox: Trivial cleanup

Move 'basepython' to the top-level 'testenv'.
Use the default 'install_command'

https://tox.readthedocs.io/en/latest/config.html#conf-install_command

Change-Id: I792f90f68c1f9f9848445c8ba8f9f7a207806fc5

tox.ini

@@ -1,22 +1,21 @@
 [tox]
-minversion = 2.0
+minversion = 3.1
 envlist = docs
 skipsdist = True
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 usedevelop = True
-install_command = pip install -U {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:docs]
-basepython = python3
 whitelist_externals =
   rm
 deps =