tox.ini: Install a py3-dev environment to ease development

Failing tests is less noisy in development mode.

tox.ini

@@ -10,6 +10,13 @@ commands =
   pytest --cov={envsitepackagesdir}/swh/lister/ --cov-branch \
          {envsitepackagesdir}/swh/lister/ {posargs}
 
+[testenv:py3-dev]
+deps =
+  swh.core[http] >= 0.0.61
+  .[testing]
+  pytest-cov
+commands =
+  pytest {envsitepackagesdir}/swh/lister/ {posargs}
 
 [testenv:flake8]
 skip_install = true