Fix import

duniterpy/documents/ws2p/heads.py

 import attr
 import re
 
+from ..document import  MalformedDocumentError
 from ..block import BlockUID
 from ..constants import ws2p_public_prefix_regex, ws2p_private_prefix_regex,\
     pubkey_regex, signature_regex, ws2pid_regex, block_uid_regex, ws2p_head_regex