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

Inherit the jobs requirements to simplify maintainance.

Also remove pypy from tox since it's not official supported anymore
and no target exist for it.

Change-Id: I80e95eeff84d9a1a28ee3507db82ae33d2ebb937

tox.ini

@@ -1,6 +1,6 @@
 [tox]
-minversion = 3.1
-envlist = py37,pypy,pep8
+minversion = 3.2.0
+envlist = py37,pep8
 ignore_basepython_conflict = True
 
 [testenv]
@@ -21,8 +21,7 @@ commands =
   {toxinidir}/tools/setup-etcd-env.sh pifpaf -e OSLO_CACHE_TEST run etcd --  stestr run --slowest functional.*
 
 [testenv:pep8]
-deps =
-  -r{toxinidir}/test-requirements.txt
+deps = {[testenv]deps}
 commands =
   flake8
   # Run security linter
@@ -34,7 +33,7 @@ commands = {posargs}
 [testenv:docs]
 whitelist_externals = rm
 deps =
-  {[testenv]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
@@ -61,7 +60,7 @@ import_exceptions =
 [testenv:releasenotes]
 whitelist_externals =
   rm
-deps = -r{toxinidir}/doc/requirements.txt
+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