Pin pylint version

Pins the version of pylint to 2.6.0, which corrects the upstream
dependency on SortImports.

Change-Id: I6d3a08ab34e62c03e119f84f66452ec4934c19c4
Signed-off-by: beierlm <mark.beierl@canonical.com>
diff --git a/tox.ini b/tox.ini
index a0f0503..4d64762 100644
--- a/tox.ini
+++ b/tox.ini
@@ -34,7 +34,7 @@
 [testenv:pylint]
 basepython = python3
 deps =
-  pylint
+  pylint==2.6.0
   -rrequirements.txt
 commands =
   pylint -E n2vc --ignore=tests
@@ -42,7 +42,7 @@
 [testenv:pylint-tests]
 basepython = python3
 deps =
-  pylint
+  pylint==2.6.0
   -rrequirements.txt
   -rtest-requirements.txt
 commands =