[enh] Use static method to load private key file

duniterpy/key/signing_key.py

@@ -68,7 +68,6 @@ class SigningKey(libnacl.sign.Signer):
 
         return cls(seed)
 
-
     def save_private_key(self, path: str) -> None:
         """
         Save authentication file
@@ -77,7 +76,7 @@ class SigningKey(libnacl.sign.Signer):
         """
         self.save(path)
 
-
+    @staticmethod
     def from_private_key(path: str) -> SigningKeyType:
         """
         Read authentication file
@@ -89,7 +88,6 @@ class SigningKey(libnacl.sign.Signer):
         key.pubkey = Base58Encoder.encode(key.vk)
         return key
 
-
     def decrypt_seal(self, message: bytes) -> str:
         """
         Decrypt message with a curve25519 version of the ed25519 key pair