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Python uses serverAuth and clientAuth in the exact same meaning as EKU (extended key usage). In order to create X.509 cert for a web server, it should have EKU "SSL/TLS Web Server Authentication". On the other hand a client must validate the cert for a specific purpose, too. So the client creates a context with purpose SERVER_AUTH. This loads only trust anchors which are flagged with EKU "SSL/TLS Web Server Authentication".
For TLS/SSL server it is the other way around. The server side uses a context with CLIENT_AUTH to load only root certs that can validate client certs. Other purposes aren't supported because Python's ssl does neither support S/MIME nor code signing.
https://docs.python.org/2/library/ssl.html#ssl.SSLContext.load_default_certs explains the purpose flags, too.