[tox] add a new command to test from-forge

tox.ini

 [tox]
-envlist=py3,check-manifest,flake8,mypy,black,black-run
+envlist=py3,py3-from-forge,check-manifest,flake8,mypy,black,black-run
 
 [testenv]
 basepython=python3
@@ -7,6 +7,16 @@ deps =
   pytest
 commands={envpython} -m pytest {posargs:{toxinidir}/test}
 
+[testenv:py3-from-forge]
+basepython=python3
+deps =
+  pytest
+  hg-evolve  # to hide some warnings
+commands =
+  pip install hg+https://forge.extranet.logilab.fr/open-source/logilab-common
+  pip install hg+https://forge.extranet.logilab.fr/open-source/logilab-database
+  {envpython} -m pytest {posargs:{toxinidir}/test}
+
 [testenv:check-manifest]
 skip_install = true
 deps =