Add support for public keys in OpenSSH (RFC 4253) format.

Cryptography previously lacked support for ECDSA keys in RFC 4253
format. Now that they have support for those keys, we should take
advantage of it and support them in PyJWT.

Implements #243.

jwt/algorithms.py

@@ -356,7 +356,10 @@ def prepare_key(self, key):
                 # a Signing Key or a Verifying Key, so we try
                 # the Verifying Key first.
                 try:
-                    key = load_pem_public_key(key, backend=default_backend())
+                    if key.startswith(b'ecdsa-sha2-'):
+                        key = load_ssh_public_key(key, backend=default_backend())
+                    else:
+                        key = load_pem_public_key(key, backend=default_backend())
                 except ValueError:
                     key = load_pem_private_key(key, password=None, backend=default_backend())