Make the default testenv target more generic

Instead of going back to py3, pass through the environment name, so that
it can be called with an arbitrary interpreter version.

tox.ini

@@ -23,10 +23,10 @@ commands =
   github: {envsitepackagesdir}/swh/core/github/tests \
     {posargs}
 
-[testenv:py3]
+[testenv:py3{,7,8,9,10,11,12,13},pypy3{,7,8,9,10,11,12,13}]
 skip_install = true
-deps = tox
-commands = tox run -e py3-core-db-server-github-slow-cover -- {posargs}
+allowlist_externals = tox
+commands = tox run -e {env_name}-core-db-server-github-slow-cover -- {posargs}
 
 [testenv:black]
 skip_install = true