tox: workaround to pip's inability to properly solve dependency resolution

This is (hopefully) a temporary fix that can be removed as soon as
  https://github.com/pypa/pip/issues/6239
is fixed, probably thanks to
  https://github.com/pypa/pip/issues/988

tox.ini

@@ -3,6 +3,7 @@ envlist=flake8,py3
 
 [testenv:py3]
 deps =
+  swh.core[http] >= 0.0.61
   .[testing]
   pytest-cov
 commands =