KeyServerPreferences is a bitfield, not a list

Key Server Preferences (RFC 4880 §5.2.3.17) is a bitfield, more like
Key Flags (RFC 4880 §5.2.3.21) than Preferred Hash Algorithms (RFC
4880 §5.2.3.8).

The caller should be able to invoke this as a set when calling
PGPKey.certify().

This patch also improves documentation for PGPKey.certify() to
indicate how to pass in these flags.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>

pgpy/constants.py

@@ -433,8 +433,7 @@ class SignatureType(IntEnum):
     ThirdParty_Confirmation = 0x50
 
 
-class KeyServerPreferences(IntEnum):
-    Unknown = 0x00
+class KeyServerPreferences(FlagEnum):
     NoModify = 0x80
 
 

pgpy/packet/subpackets/signature.py

@@ -690,7 +690,7 @@ class PreferredCompressionAlgorithms(FlagList):
     __flags__ = CompressionAlgorithm
 
 
-class KeyServerPreferences(FlagList):
+class KeyServerPreferences(ByteFlag):
     __typeid__ = 0x17
     __flags__ = _KeyServerPreferences
 

pgpy/pgp.py

@@ -1927,6 +1927,8 @@ class PGPKey(Armorable, ParentRef, PGPObject):
         :keyword keyserver: Specify the URI of the preferred key server of the user.
                             This keyword is ignored for non-self-certifications.
         :type keyserver: ``str``, ``unicode``, ``bytes``
+        :keyword keyserver_flags: A set of Key Server Preferences, as :py:obj:`~constants.KeyServerPreferences`.
+        :type keyserver_flags: ``set``
         :keyword primary: Whether or not to consider the certified User ID as the primary one.
                           This keyword is ignored for non-self-certifications, and any certifications directly on keys.
         :type primary: ``bool``