Added possibility to call jwk.construct() with a private key (#295)

Co-authored-by: DataGhost <[email protected]>

Author: DataGhost

jose/backends/cryptography_backend.py

@@ -228,8 +228,8 @@ def __init__(self, key, algorithm, cryptography_backend=default_backend):
 
         self.cryptography_backend = cryptography_backend
 
-        # if it conforms to RSAPublicKey interface
-        if hasattr(key, "public_bytes") and hasattr(key, "public_numbers"):
+        # if it conforms to RSAPublicKey or RSAPrivateKey interface
+        if (hasattr(key, "public_bytes") and hasattr(key, "public_numbers")) or hasattr(key, "private_bytes"):
             self.prepared_key = key
             return