Adjust tox.ini for tox4

OpenStack CI now uses tox4 by default.

- Drop "skipsdist = True".
  If skipsdist is specified, horizon is not installed into tox env.
- Env vars specified in passenv should be one per line (or separated by
  comma) in tox4.

Change-Id: I60d2db52e9aac580859de3ed17fde14f9fd262a2

tox.ini

@@ -1,7 +1,6 @@
 [tox]
 minversion = 3.18.0
 envlist = pep8,py39,releasenotes,npm,py3-dj32
-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.
@@ -121,7 +120,14 @@ commands =
 # Provide an environment for system packagers that dont want anything from pip
 # Any extra deps needed for this env can be passed by setting TOX_EXTRA_DEPS
 sitepackages = True
-passenv = TOX_EXTRA_DEPS http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
+passenv =
+  TOX_EXTRA_DEPS
+  http_proxy
+  HTTP_PROXY
+  https_proxy
+  HTTPS_PROXY
+  no_proxy
+  NO_PROXY
 # Sets deps to an empty list so nothing is installed from pip
 deps =
 commands =