Update tox config

Update tox min version to something more contemporary.
Default basepython to python3.
Update dependencies of docs and pdf-docs to include upper-constraints.
Move to stestrs and remote os-test from requirements.

Change-Id: I30313dda1a3f56eff2f54632c46cfbdd03aa3cb7

test-requirements.txt

@@ -4,5 +4,5 @@
 
 doc8 # Apache-2.0
 stestr!=2.3.1 # Apache-2.0
-os-testr>=1.0.0 # Apache-2.0
 testtools>=0.9.34
+

tox.ini

@@ -1,30 +1,37 @@
 [tox]
-minversion = 2.0
+minversion = 3.9.0
 envlist = linters,docs
-skipsdist = True
+skipsdist = true
+ignore_basepython_conflict = true
 
 [testenv]
 usedevelop = True
+basepython = python3
 setenv =
    VIRTUAL_ENV={envdir}
 deps =
-    -r{toxinidir}/requirements.txt
-    -r{toxinidir}/test-requirements.txt
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/requirements.txt
+  -r{toxinidir}/test-requirements.txt
 commands =
-    ostestr {posargs}
+    stestr run {posargs}
     doc8 priorities/ specs/ doc/source README.rst
 
 [testenv:venv]
-basepython = python3
-commands = {posargs}
+commands = {posargs:}
 
 [testenv:docs]
-basepython = python3
-commands = sphinx-build -W -b html doc/source doc/build/html
+sitepackages = False
+deps =
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/requirements.txt
+commands =
+  sphinx-build -b html doc/source doc/build/html
 
 [testenv:pdf-docs]
-basepython = python3
 whitelist_externals = make
-commands = sphinx-build -W -b latex doc/source doc/build/pdf
-           make -C doc/build/pdf
-
+sitepackages = False
+deps = {[testenv:docs]deps}
+commands =
+  sphinx-build -b latex doc/source doc/build/pdf
+  make -C doc/build/pdf