tox: Set 'ignore_basepython_conflict'

Ensure we run with the versions of Python we expect to. Some other
cruft is removed.

Signed-off-by: Stephen Finucane <stephenfin@redhat.com>
Change-Id: I450c062eed1185e7e4c49a156307abb4320f6cb6

tox.ini

@@ -1,14 +1,12 @@
 [tox]
-minversion = 2.0
+minversion = 3.1
 envlist = py37,pypy,pep8
 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
 commands = python setup.py testr --slowest --testr-args='{posargs}'