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/.azure-pipelines.yml b/.azure-pipelines.yml
index 0f1a1bd..cd54688 100644
--- a/.azure-pipelines.yml
+++ b/.azure-pipelines.yml
@@ -214,7 +214,7 @@
           cd ${WORK_DIR}
           export USER=azure
           pip install -r test/py/requirements.txt
-          pip install asteval pylint pyopenssl
+          pip install asteval pylint==2.12.2 pyopenssl
           export PATH=${PATH}:~/.local/bin
           echo "[MASTER]" >> .pylintrc
           echo "load-plugins=pylint.extensions.docparams" >> .pylintrc