add support for Brainpool curves in TLS 1.3 (RFC8734)

[tomato42]

Add support for Brainpool curves in TLS 1.3, both for ECDH and signing

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This change is 

[ueno]

Feature-wise, it looks good to me, though I see quite a lot of if curve_name == ... for those curves; perhaps they could be a dict lookup and defined in a single place for simplicity?

[tomato42]

Feature-wise, it looks good to me, though I see quite a lot of if curve_name == ... for those curves; perhaps they could be a dict lookup and defined in a single place for simplicity?

yes, I was thinking of this, but I think it will be better to combine that with the refactoring of the all the different places that do signing and verification