Update pylint target for python3 and upper constraints

This change imposes the upper constraint in tox to protect from
future releases causing random breakage.

Ex: A new version of python-libvirt was released Jan 23 2020
which will not install on python2.

This change also enables the python3 target for pylint which will
allow the upper constraint to be changed to a more recent version
when all the tox files are aligned.

Change-Id: I9056778085d32b3401df60c20d67cff0a21dfe97
Story: 2004515
Task: 38496
Signed-off-by: Al Bailey <Al.Bailey@windriver.com>
(cherry picked from commit 7b7959e9b5)

tox.ini

@@ -5,7 +5,9 @@ skipsdist = True
 stxdir = {toxinidir}/..
 
 [testenv]
-install_command = pip install -U {opts} {packages}
+install_command = pip install -U \
+  -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/stable/stein/upper-constraints.txt} \
+  {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
          OS_STDOUT_CAPTURE=1
          OS_STDERR_CAPTURE=1
@@ -78,7 +80,7 @@ commands =
     flake8
 
 [testenv:pylint]
-basepython = python2.7
+basepython = python3
 deps = -r{toxinidir}/test-requirements.txt
        -e{[tox]stxdir}/config/tsconfig/tsconfig
        -e{[tox]stxdir}/fault/fm-api