ci(pytest): pin pytest version at 6.2.5

After a major version release (7.0.0), our current FVT tests all
failed due to some change in the lib.  Our pytest version in CI was
not pinned, and picked this update up.

.ci/docker-compose-file/python/pytest.sh

@@ -19,7 +19,7 @@ fi
 
 apk update && apk add git curl
 git clone -b develop-4.0 https://github.com/emqx/paho.mqtt.testing.git /paho.mqtt.testing
-pip install pytest
+pip install pytest==6.2.5
 
 pytest -v /paho.mqtt.testing/interoperability/test_client/V5/test_connect.py -k test_basic --host "$TARGET_HOST"
 RESULT=$?