Added a subkey binding signature subpacket.

pgpy/packet/subpackets/signature.py

@@ -52,6 +52,7 @@ __all__ = ['URI',
            'Policy',
            'KeyFlags',
            'SignersUserID',
+           'SubkeyBindingSignature',
            'ReasonForRevocation',
            'Features',
            'EmbeddedSignature',
@@ -700,8 +701,10 @@ class KeyServerPreferences(ByteFlag):
 class PreferredKeyServer(URI):
     __typeid__ = 0x18
 
+class SubkeyBindingSignature(Signature):
+    __typeid__ = 0x18
 
-class PrimaryUserID(Signature):
+class PrimaryUserID(SubkeyBindingSignature):
     __typeid__ = 0x19
 
     @sdproperty