Fix pylint version constraint

The pep8 tox environment was failing with numerous warnings when
pylint>=4.0.0. Looks an issue on the pylint library [0]

[0] pylint-dev/pylint#10669

Change-Id: Iae8f113944e597c4a4eadc14b31c405970898626
Signed-off-by: Fernando Royo <[email protected]>

Author: fernandoroyosanchez

test-requirements.txt

@@ -5,7 +5,7 @@ coverage!=4.4,>=4.0 # Apache-2.0
 flake8-import-order>=0.18.0,<0.19.0 # LGPLv3
 oslotest>=3.2.0 # Apache-2.0
 stestr>=1.0.0 # Apache-2.0
-pylint>=2.6.0 # GPLv2
+pylint>=2.6.0,!=4.0.0,!=4.0.1 # GPLv2
 testresources>=2.0.0 # Apache-2.0/BSD
 testscenarios>=0.4 # Apache-2.0/BSD
 WebTest>=2.0.27 # MIT