Fix support for enabling Brainpool curves in SslClient

[nabla-c0d3]

Brainpool-related constants have been added to nassl, but right now they can only be used to detect when a server is using Brainpool curves.

Trying to enable the Brainpool curves using SslClient.set_groups() currently triggers an OpenSslError: 662d1f4#diff-5a72a6ef501b66068212269ba8bc698006849888af05c7aaabe83a5197ae84c9R21