Sync the tox.ini files with the other plugins

Missed during the reviews process of the splitting. Main changes:

- envlist for py37 too;
- minversion bumped to 2.0;
- fixed the wrong basepython statement for for 'testenv' tox target;
- consider the expected dependencies in the 'pep8' and 'releasenotes'
  tox targets;
- add the 'lower-constraints' tox target

Exception: do not sync the ignore rules for pep8, as they
are apparently not needed.

Change-Id: I2297ed7b0b0fc1205feeff5686ec08439d7c814d

tox.ini

@@ -1,10 +1,9 @@
 [tox]
-envlist = py35,py27,pep8
-minversion = 1.6
+envlist = py36,py35,py27,pep8
+minversion = 2.0
 skipsdist = True
 
 [testenv]
-basepython = python3
 usedevelop = True
 install_command = pip install {opts} {packages}
 setenv =
@@ -27,6 +26,11 @@ commands = oslo_debug_helper -t sahara_plugin_spark/tests/unit {posargs}
 
 [testenv:pep8]
 basepython = python3
+deps =
+  -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
+  -r{toxinidir}/requirements.txt
+  -r{toxinidir}/test-requirements.txt
+  -r{toxinidir}/doc/requirements.txt
 commands =
     flake8 {posargs}
     doc8 doc/source
@@ -48,7 +52,13 @@ whitelist_externals =
 
 [testenv:releasenotes]
 basepython = python3
-commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
+deps =
+  -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
+  -r{toxinidir}/doc/requirements.txt
+commands =
+  rm -rf releasenotes/build releasenotes/html
+  sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
+whitelist_externals = rm
 
 [testenv:debug]
 basepython = python3
@@ -69,3 +79,9 @@ exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,tools
 # [H205] Use assert(Greater|Less)(Equal) for comparison
 enable-extensions=H904,H106,H203,H204,H205
 
+[testenv:lower-constraints]
+basepython = python3
+deps =
+  -c{toxinidir}/lower-constraints.txt
+  -r{toxinidir}/test-requirements.txt
+  -r{toxinidir}/requirements.txt