Update and clean tox configuration for version 4

Related to swh/meta#4959

tox.ini

 [tox]
+requires =
+  tox>4
 envlist=black,flake8,mypy,java,py3
 
 [testenv]
@@ -59,8 +61,7 @@ extras =
   luigi
 deps =
   # fetch and install swh-docs in develop mode
-  -e git+https://forge.softwareheritage.org/source/swh-docs#egg=swh.docs
-
+  -e git+https://gitlab.softwareheritage.org/swh/devel/swh-docs.git\#egg=swh.docs
 setenv =
   SWH_PACKAGE_DOC_TOX_BUILD = 1
   # turn warnings into errors
@@ -68,7 +69,6 @@ setenv =
 commands =
   make -I ../.tox/sphinx/src/swh-docs/swh/ -C docs
 
-
 # build documentation only inside swh-environment using local state
 # of swh-docs package
 [testenv:sphinx-dev]
@@ -80,7 +80,6 @@ extras =
 deps =
   # install swh-docs in develop mode
   -e ../swh-docs
-
 setenv =
   SWH_PACKAGE_DOC_TOX_BUILD = 1
   # turn warnings into errors