My connection via PyMongo suddenly stopped working on Sept. 29, 2021. It seems problem is related to MongoDB Atlas certificates. I had to use this workaround:
https://www.mongodb.com/community/forums/t/m220-connection-issue-trying-to-connect-with-mongo-shell-ssl-certificate-verify-failed/104064/6
May MongoDB Atlas admins announce such events explicitly and/or in advance :S