tox: Add support for tox 4.x

Some deprecated tox 3.x features have been removed in tox 4.x
and some install behaviors changed between the two versions.

So add tox 4.x support while keeping tox >= 3.7 compatibility.

tox.ini

@@ -3,7 +3,7 @@ envlist=flake8,black
 
 [testenv]
 basepython = python3
-passenv = SPHINXOPTS SPHINXOPTCOLOR JAVA_HOME
+passenv = SPHINXOPTS,SPHINXOPTCOLOR,JAVA_HOME
 
 [testenv:sphinx]
 extras =
@@ -19,13 +19,12 @@ commands =
   {envpython} -m pifpaf run postgresql -- make -C docs {posargs:html}
 
 [testenv:sphinx-dev]
-whitelist_externals = make
-# workaround really long dependencies installation due to the
-# new pip resolver and its backtracking processes
-install_command = make pip-install-swh-dev {packages}
+allowlist_externals = make
 setenv =
   SWHPKGDIR = {envsitepackagesdir}/swh
 commands =
+  # use custom install script for dev build
+  make pip-install-swh-dev
   {envpython} -m pifpaf run postgresql -- make -C sysadm {posargs:html}
   {envpython} -m pifpaf run postgresql -- make -C user {posargs:html}
   {envpython} -m pifpaf run postgresql -- make -C docs {posargs:html}