Thanks for the patch. Some preliminary comments:

- the OpenSSL functions you are using (SSL_get_srp_username etc.) don't seem documented on openssl.org; this makes it harder to do a proper review
- no need to fill Misc/ACKS and Misc/NEWS by yourself, we can take care of that
- what is an "SRP vbase"? is it something standardized, or OpenSSL-specific?
- if server-side support needs a callback, I think it would be better to let users write their callback in Python, rather than force a hardwired implementation
- ssl.wrap_socket() is the legacy API, I would rather add new features only to the SSLContext API