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

@@ -16,12 +16,12 @@ commands =
   minimal: {envsitepackagesdir}/swh/model/tests/test_cli.py -m 'not requires_optional_deps' \
            {posargs}
 
-[testenv:py3]
+[testenv:py3{,7,8,9,10,11,12,13},pypy3{,5,6,7,8,9,10,11,12,13}]
 skip_install = true
-deps = tox
+allowlist_externals = tox
 commands =
-  tox run -e py3-full-cover -- {posargs}
-  tox run -e py3-minimal -- {posargs}
+  tox run -e {env_name}-full-cover -- {posargs}
+  tox run -e {env_name}-minimal
 
 [testenv:black]
 skip_install = true