Fix tox4 error

tox.ini started failing with Tox4 which had some
incompatible changes. One is passenv where we need
to pass each value in newline otherwise, it fails with
error:

 failed with pass_env values cannot contain whitespace,
 use comma to have multiple values in a single line,
 invalid values found.....

Other failure is due to the skipsdist = True.

Fixing tox.ini for tox4 changes.

Change-Id: Ib1e58d792ff12d3c3b72a001e04fa9b8c62d1ef8

tox.ini

@@ -1,11 +1,9 @@
 [tox]
 minversion = 3.18.0
 envlist = docs,py38,pep8
-skipsdist = True
 ignore_basepython_conflict = True
 
 [testenv]
-basepython = python3
 usedevelop = True
 setenv =
    VIRTUAL_ENV={envdir}
@@ -27,6 +25,8 @@ commands = flake8
            # Run security linter
            bandit -r octavia_lib -ll -ii -x octavia_lib/tests
            {toxinidir}/tools/coding-checks.sh --pylint '{posargs}'
+allowlist_externals =
+    {toxinidir}/tools/coding-checks.sh
 
 [testenv:venv]
 commands = {posargs}
@@ -46,7 +46,8 @@ commands =
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/doc/requirements.txt
-allowlist_externals = rm
+allowlist_externals =
+  rm
 commands =
   rm -rf doc/build doc/source/reference/modules
   sphinx-build -W -b html doc/source doc/build/html