Update requirements.txt and tox.ini

- Update to juju==2.9.4: needed for feature 10884
- Pin pylint version in tox.ini

Change-Id: Ie7115804973f97e61e9bb8946215ce958abe8089
Signed-off-by: David Garcia <david.garcia@canonical.com>
diff --git a/tox.ini b/tox.ini
index 0e3f69f..bf31bce 100644
--- a/tox.ini
+++ b/tox.ini
@@ -65,7 +65,7 @@
 deps =  {[testenv]deps}
         -r{toxinidir}/requirements-dev.txt
         -r{toxinidir}/requirements-test.txt
-        pylint
+        pylint==2.10.2
 commands =
     pylint -E n2vc