ci: Remove no-self-use exception in main_test.py
ci: Remove no-self-use exception in main_test.py

https://pylint.pycqa.org/en/latest/whatsnew/2/2.14/summary.html#removed-checkers
GoogleCloudPlatform/document-ai-samples,GoogleCloudPlatform/document-ai-samples,GoogleCloudPlatform/document-ai-samples,GoogleCloudPlatform/document-ai-samples,GoogleCloudPlatform/document-ai-samples