Make tox.ini tox 4.0.0 compatible

* removed skipsdist=True to make sure placement available in the virtual
  env

* passenv fixed as space separated list is not allowed any more

* improved the doc generation command line to print all the errors
  instead of stopping at the first one

* added functional-py310 for convenience

* removed basepython = python3 as we assume all developer switched to
  python3 in their env already

* removed ignore_basepython_conflict = True as without the basepython
  definition generative targets now work without conflict

* updated the install_command to mimic better what tox uses by default

Depends-On: https://review.opendev.org/c/zuul/zuul-jobs/+/866943
Change-Id: I1de24b7a98f149fce33d4e5eca82460eea72140b

tox.ini

@@ -1,20 +1,14 @@
 [tox]
 minversion = 3.18.0
 envlist = py39,functional-py39,pep8
-skipsdist = True
-# Automatic envs (pyXX) will use the python version appropriate to that
-# env and ignore basepython inherited from [testenv]. That's what we
-# want, and we don't need to be warned about it.
-ignore_basepython_conflict = True
 
 [testenv]
-basepython = python3
 usedevelop = True
 allowlist_externals =
   bash
   rm
   env
-install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
+install_command = python -I -m pip install -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
 setenv =
   VIRTUAL_ENV={envdir}
   LANGUAGE=en_US
@@ -30,7 +24,8 @@ deps = -r{toxinidir}/test-requirements.txt
 commands =
   stestr run {posargs}
 passenv =
-  OS_DEBUG GENERATE_HASHES
+  OS_DEBUG
+  GENERATE_HASHES
 # there is also secret magic in subunit-trace which lets you run in a fail only
 # mode. To do this define the TRACE_FAILONLY environmental variable.
 
@@ -49,6 +44,11 @@ envdir = {toxworkdir}/py39
 commands =
   {[testenv:functional]commands}
 
+[testenv:functional-py310]
+envdir = {toxworkdir}/py310
+commands =
+  {[testenv:functional]commands}
+
 [testenv:pep8]
 description =
   Run style checks.
@@ -109,7 +109,7 @@ description =
 deps = -r{toxinidir}/doc/requirements.txt
 commands =
   rm -rf doc/build
-  sphinx-build -W -b html -d doc/build/doctrees doc/source doc/build/html
+  sphinx-build -W --keep-going -b html -j auto doc/source doc/build/html
   # Test the redirects
   whereto doc/build/html/.htaccess doc/test/redirect-tests.txt
 
@@ -163,9 +163,9 @@ max-complexity=19
 # system dependencies are missing, since it's used to tell you what system
 # dependencies are missing! This also means that bindep must be installed
 # separately, outside of the requirements files, and develop mode disabled
-# explicitly to avoid unnecessarily installing the checked-out repo too (this
-# further relies on "tox.skipsdist = True" above).
+# explicitly to avoid unnecessarily installing the checked-out repo too
 usedevelop = False
+skipsdist = True
 deps = bindep
 commands =
   bindep test