Feature:: Support for keys without usage flags (#371)

Co-authored-by: James Morris <jmorris@securityinnovation.com>

pgpy/decorators.py

@@ -93,7 +93,11 @@ class KeyAction(object):
                     break
 
             else:  # pragma: no cover
-                raise PGPError("Key {keyid:s} does not have the required usage flag {flags:s}".format(**em))
+                warning = "Key {keyid:s} does not have the required usage flag {flags:s}".format(**em)
+                if key._require_usage_flags:
+                    raise PGPError(warning)
+                else:
+                    logging.warning(warning)
 
         else:
             _key = key

pgpy/pgp.py

@@ -1618,6 +1618,7 @@ class PGPKey(Armorable, ParentRef, PGPObject):
         self._signatures = SorteDeque()
         self._uids = SorteDeque()
         self._sibling = None
+        self._require_usage_flags = True
 
     def __bytearray__(self):
         _bytes = bytearray()