Update tox.ini for tox4

With tox4, we need to pass the environment variables on separate lines as opposed to separated by whitespace. The skipdist setting also breaks the runs.

Change-Id: Iad13eb46c2cd89169c1aa9c39492ef7a29c83161

tox.ini

@@ -1,14 +1,12 @@
 [tox]
 envlist = py3,flake8
 minversion = 3.1.0
-skipsdist = True
 skip_missing_interpreters = true
 # this allows tox to infer the base python from the environment name
 # and override any basepython configured in this file
 ignore_basepython_conflict = true
 
 [testenv]
-basepython = python3
 usedevelop = True
 install_command = pip install {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
@@ -20,14 +18,20 @@ deps =
        -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 
-whitelist_externals = find
+allowlist_externals = find
                       sh
                       rm
 
 commands =
   find . -type f -name "*.pyc" -delete
   stestr run --slowest {posargs}
-passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
+passenv =
+  http_proxy
+  HTTP_PROXY
+  https_proxy
+  HTTPS_PROXY
+  no_proxy
+  NO_PROXY
 
 [testenv:docs]
 deps =