Fixes #1533 -- Initial work at parsing ECDSA public keys in OpenSSH format

Are there any other types of keys in common use besides Ed25519? (This will
affect how I word the docs)

On Wed Dec 24 2014 at 3:41:29 PM Mohammed Attia <notifications@github.com>
wrote:

> In docs/hazmat/primitives/asymmetric/serialization.rst
> <https://github.com/pyca/cryptography/pull/1576#discussion-diff-22264990>:
>
> > @@ -201,9 +201,6 @@ OpenSSH Public Key
> >
> >  The format used by OpenSSH to store public keys, as specified in :rfc:`4253`.
> >
> > -Currently, only RSA and DSA public keys are supported. Any other type of key
> > -will result in an exception being thrown.
> > -
>
> I think this should remain the same and add EcDSA to it, or just specify
> that we still don't support ed25519 at the moment?
>
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> <https://github.com/pyca/cryptography/pull/1576/files#r22264990>.
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