trivial: Cleanup tox.ini

Update the minversion parameter to use the python -m pip to install
python packages:

https://tox.readthedocs.io/en/latest/changelog.html#id185

Also Inherit the jobs requirements to simplify maintainance.

Change-Id: I730f15a4872334f040d5bd03da37e7c4f9e8148b

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.1
+minversion = 3.2.0
 distribute = False
 envlist = py37,pep8
 ignore_basepython_conflict = true
@@ -51,7 +51,9 @@ commands = {posargs}
 
 [testenv:docs]
 whitelist_externals = rm
-deps = -r{toxinidir}/doc/requirements.txt
+deps =
+  -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
+  -r{toxinidir}/doc/requirements.txt
 commands =
   rm -fr doc/build
   sphinx-build -W --keep-going -b html doc/source doc/build/html {posargs}
@@ -65,8 +67,11 @@ enable-extensions = H203,H106
 exclude = .tox,dist,doc,*.egg,build
 
 [testenv:releasenotes]
-deps = -r{toxinidir}/doc/requirements.txt
+whitelist_externals =
+  rm
+deps = {[testenv:docs]deps}
 commands =
+  rm -rf releasenotes/build
   sphinx-build -a -E -W -d releasenotes/build/doctrees --keep-going -b html releasenotes/source releasenotes/build/html
 
 [hacking]