bpo-41195: Add getter for Openssl security level (GH-21282)

Add an accessor under SSLContext.security_level as a wrapper around
SSL_CTX_get_security_level, see:
https://www.openssl.org/docs/manmaster/man3/SSL_CTX_get_security_level.html


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This is my first time contributing, so please pull me up on all the things I missed or did incorrectly.

Automerge-Triggered-By: @tiran