CI: Pin pylint version to 2.12.2

For consistency in runs, we need to always use the same pylint version.
Pin to 2.12.2 as this is what we have been using so far.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 388e666..7df7e93 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -220,7 +220,7 @@
   stage: testsuites
   script:
     - pip install -r test/py/requirements.txt
-    - pip install asteval pylint pyopenssl
+    - pip install asteval pylint==2.12.2 pyopenssl
     - export PATH=${PATH}:~/.local/bin
     - echo "[MASTER]" >> .pylintrc
     - echo "load-plugins=pylint.extensions.docparams" >> .pylintrc