Remove virtualenv pinning

virtualenv pinning was added on Yoga final release time because some of
the deliverables' packaging didn't work with the new python module
discovery (see details: [1]). 1 year passed, hopefully most of the
projects have fixed these errors, so let's unpin virtualenv.

Since newest virtualenv introduces some errors, those need to be fixed:
- skipdist=False needs to be removed otherwise openstack_releases won't
  be installed
- basepython=python3 is not necessary anymore since there is no py2
- allowlist_external is needed for tools/* scripts
This patch also adds python3.10 classifier to setup.cfg

[1] https://review.opendev.org/835423

Change-Id: Ibc1f538ddd04c0afeb3d5464fe0b1347ee3ea67a

setup.cfg

@@ -16,6 +16,7 @@ classifier =
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
+    Programming Language :: Python :: 3.10
 
 [files]
 packages = openstack_releases

tox.ini

@@ -1,13 +1,6 @@
 [tox]
 minversion = 3.2.0
 envlist = py3,validate,pep8,bashate,docs
-skipsdist = True
-ignore_basepython_conflict=true
-# note(elod.illes): setuptools capping / pinning does not work as it is
-# pinned inside virtualenv. So to cap / pin setuptools we need to do it
-# via virtualenv. Virtualenv 20.13.3 is the last version that holds
-# setuptools < 61.0.0
-requires = virtualenv==20.13.3
 
 [testenv]
 usedevelop=True
@@ -24,7 +17,8 @@ setenv =
    OS_STDERR_CAPTURE=1
    OS_DEBUG=1
    OS_LOG_CAPTURE=1
-basepython = python3
+allowlist_externals =
+    {toxinidir}/tools/*
 deps =
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/test-requirements.txt