Bump pylint to 2.6.0

The current version does not work anymore with the current
testing stack.
Due to the bump, the wrapper needed a few changes
(maybe it could be dropped at some point?

Version 2.3.0 (used by cinder right now) was considered too,
but it requires a specific version of isort:
https://github.com/PyCQA/isort/issues/1273

Change-Id: I07fa32e7f49bde041d101a2d09860d0bc27acda0

tools/lintstack.py

@@ -153,7 +153,7 @@ def run_pylint():
     args = [
         "--msg-template='{path}:{line}: [{msg_id}({symbol}), {obj}] {msg}'",
         "-E", "cinderclient"]
-    lint.Run(args, reporter=reporter, exit=False)
+    lint.Run(args, reporter=reporter, do_exit=False)
     val = buff.getvalue()
     buff.close()
     return val

tox.ini

@@ -38,7 +38,7 @@ commands =
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/requirements.txt
-       pylint==1.9.1
+       pylint==2.6.0
 commands = bash tools/lintstack.sh
 whitelist_externals = bash