[tox] Update constraints, flake8 settings and default envlist

In validating the change after this I found that:
1. The pypy tox env defaults to pypy2 which obviously is a problem
2. The constraints URL, while functional, follows an older more complex form
3. The pep8 tox env would descend into .tox which fails as packages
   outside of out control may have pep8 issues
4. I often have a .venv directory which fails for the same reason as
   point 3

This change addresses those issues suck that a `tox` run completes as
expected.

Change-Id: Ia64b5f7b69744ede12604bfd204ae0cbdb6aeea2

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py3,pypy,pep8
+envlist = py3,pypy3,pep8
 minversion = 3.1
 skipsdist = True
 ignore_basepython_conflict = True
@@ -13,7 +13,7 @@ setenv = VIRTUAL_ENV={envdir}
          OS_STDERR_NOCAPTURE=False
          PYTHONHASHSEED=0
 deps =
-       -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
+       -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = stestr run {posargs}
@@ -38,3 +38,6 @@ deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt
   -r{toxinidir}/requirements.txt
+
+[flake8]
+exclude=.tox,.venv