Cap pycodestyle to < 2.6.0

version 2.6.0 introduced new check that now fail on older code branches.
There's no point in adding style changes to old code, cap pycodestyle
instead.

Related-Prod: PRODX-3456
Change-Id: If2619e9e2fe30f71209eebae96d3a9b64499f781
diff --git a/test-requirements.txt b/test-requirements.txt
index a50905f..565198b 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -5,4 +5,5 @@
 mock>=2.0.0 # BSD
 coverage!=4.4,>=4.0 # Apache-2.0
 oslotest>=3.2.0 # Apache-2.0
+pycodestyle>=2.0.0,<2.6.0 # MIT
 flake8-import-order==0.11 # LGPLv3