[fix] #162: Fix cyclic import issue in ascii_armor

Found after imports sorting

duniterpy/key/ascii_armor.py

@@ -22,7 +22,9 @@ from typing import Any, Dict, List, Optional
 import libnacl
 from libnacl.version import version as libnacl_version
 
-from duniterpy.key import PublicKey, SigningKey, VerifyingKey
+from .encryption_key import PublicKey
+from .signing_key import SigningKey
+from .verifying_key import VerifyingKey
 
 # Headers constants
 BEGIN_MESSAGE_HEADER = "-----BEGIN DUNITER MESSAGE-----"