Fix tox to ignore_basepython_conflict

Tox includes a couple of enhancements:

- [1] previously mishandled the basepython directive.
- previously ignored PYTHONDONTWRITEBYTECODE [2]

So we can remove the workarounds.

See ML thread [3] for some details

[1] https://github.com/tox-dev/tox/issues/477
[2] 336f4f6bd8
[3] http://lists.openstack.org/pipermail/openstack-dev/2018-July/thread.html#132075

Change-Id: Ibd8b77bb10c5a739fea11a02e959b66c5e81f221
Signed-off-by: Goutham Pacha Ravi <gouthampravi@gmail.com>

tox.ini

@@ -3,19 +3,22 @@ distribute = False
 envlist = py3,pep8
 minversion = 3.18.0
 skipsdist = True
+# Automatic envs (pyXX) will only use the python version appropriate to that
+# env and ignore basepython inherited from [testenv] if we set
+# ignore_basepython_conflict.
+ignore_basepython_conflict = true
 
 [testenv]
 basepython = python3
+usedevelop = true
 setenv = VIRTUAL_ENV={envdir}
          OS_STDOUT_CAPTURE=1
          OS_STDERR_CAPTURE=1
-
-allowlist_externals = find
+         PYTHONDONTWRITEBYTECODE=1
 deps =
        -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/test-requirements.txt
 commands =
-  find . -type f -name "*.pyc" -delete
   stestr run {posargs}
   stestr slowest