tox: Declare bash as allowlist_externals

tox4 checks allowlist_externals more strictly and
all externals need to be specified explicitly.

skipsdist=True is also droppped. When skipsdist=True is specified,
a target application is not installed into a tox env, but there
seems no reason to do so. In various other OpenStack projects,
a target applicatin needs to be installed into a tox vnev, for
example, in docs and/or linter jobs and it causes failures with tox4.
I believe dropping it would avoid potential future failures.

Change-Id: I56d2a654f2be87e98026dfa237d5c81d5fcefa37

tox.ini

@@ -1,7 +1,6 @@
 [tox]
 minversion = 3.18.0
 envlist = py39,pep8
-skipsdist = True
 ignore_basepython_conflict = true
 
 [testenv]
@@ -12,6 +11,8 @@ setenv =
 deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
+allowlist_externals =
+  /bin/bash
 commands =
   /bin/bash run_tests.sh -N --no-pep8 {posargs}