Add scour dependency in sphinx and sphinx-dev tox environments

This is now used (esp. by swh-alter but it should be generalized) to
process svg images for publication.

tox.ini

@@ -19,6 +19,7 @@ extras =
 deps =
   django < 3
   pifpaf
+  scour
 setenv =
   SWHPKGDIR = {envsitepackagesdir}/swh
 commands =
@@ -29,6 +30,8 @@ allowlist_externals = make
 skip_install = true
 setenv =
   SWHPKGDIR = {envsitepackagesdir}/swh
+deps =
+  scour
 commands =
   # use custom install script for dev build
   make pip-install-swh-dev