Adapt to tox4 This fixes the several problems/errors caused by tox 4.

Change-Id: Iff615b47c1ae2ea38fb3186ee0f002fe78f49dc9

doc/requirements.txt

@@ -1,2 +1,3 @@
 openstackdocstheme>=2.2.1 # Apache-2.0
 sphinx>=2.0.0,!=2.1.0 # BSD
+yasfb>=0.8.0

tox.ini

@@ -1,6 +1,6 @@
 [tox]
-minversion = 1.6
-envlist = docs,py27
+minversion = 3.18.0
+envlist = docs,py3
 skipsdist = True
 
 [testenv]
@@ -10,7 +10,11 @@ deps = -r{toxinidir}/requirements.txt
 commands =
     stestr run {posargs}
     stestr slowest
-passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
+passenv =
+    http_proxy
+    https_proxy
+    no_proxy
+
 
 [testenv:venv]
 basepython = python3
@@ -18,12 +22,15 @@ commands = {posargs}
 
 [testenv:docs]
 basepython = python3
-deps = -r{toxinidir}/doc/requirements.txt
+deps =
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/doc/requirements.txt
 commands =
     rm -rf doc/build/html
     sphinx-build -W -b html doc/source doc/build/html
 whitelist_externals =
     rm
+allowlist_externals = rm
 
 [testenv:pdf-docs]
 basepython = python3
@@ -35,6 +42,9 @@ commands =
 whitelist_externals =
     make
     rm
+allowlist_externals =
+    make
+    rm
 
 [testenv:doc8]
 basepython = python3