[mod] Use pathlib module in exemples and a test

- https://docs.python.org/3/library/pathlib.html#correspondence-to-tools-in-the-os-module

examples/save_and_load_private_key_file.py

 from duniterpy.key import SigningKey
 import getpass
 import os
+from pathlib import Path
 
 if "XDG_CONFIG_HOME" in os.environ:
-    home_path = os.environ["XDG_CONFIG_HOME"]
+    home_path = Path(os.environ["XDG_CONFIG_HOME"])
 elif "HOME" in os.environ:
-    home_path = os.environ["HOME"]
+    home_path = Path(os.environ["HOME"])
 elif "APPDATA" in os.environ:
-    home_path = os.environ["APPDATA"]
+    home_path = Path(os.environ["APPDATA"])
 else:
-    home_path = os.path.dirname(__file__)
+    home_path = Path(__file__).parent
 
 # CONFIG #######################################
 
 # WARNING : Hide this file in a safe and secure place
 # If one day you forget your credentials,
 # you'll have to use one of your private keys instead
-PRIVATE_KEYS_FILE_PATH = os.path.join(home_path, ".duniter_account_private_keys.txt")
+PRIVATE_KEYS_FILE_PATH = Path(home_path, ".duniter_account_private_keys.txt")
 
 ################################################
 

examples/save_and_load_private_key_file_ewif.py

 from duniterpy.key import SigningKey
 import getpass
 import os
+from pathlib import Path
 
 if "XDG_CONFIG_HOME" in os.environ:
-    home_path = os.environ["XDG_CONFIG_HOME"]
+    home_path = Path(os.environ["XDG_CONFIG_HOME"])
 elif "HOME" in os.environ:
-    home_path = os.environ["HOME"]
+    home_path = Path(os.environ["HOME"])
 elif "APPDATA" in os.environ:
-    home_path = os.environ["APPDATA"]
+    home_path = Path(os.environ["APPDATA"])
 else:
-    home_path = os.path.dirname(__file__)
+    home_path = Path(__file__).parent
 
 # CONFIG #######################################
 
 # WARNING : Hide this file in a safe and secure place
 # If one day you forget your credentials,
 # you'll have to use one of your private keys instead
-PRIVATE_KEY_FILE_PATH = os.path.join(home_path, ".duniter_account_ewif_v1.duniterkey")
+PRIVATE_KEY_FILE_PATH = Path(home_path, ".duniter_account_ewif_v1.duniterkey")
 
 ################################################
 

examples/save_and_load_private_key_file_pubsec.py

 from duniterpy.key import SigningKey
 import getpass
 import os
+from pathlib import Path
 
 if "XDG_CONFIG_HOME" in os.environ:
-    home_path = os.environ["XDG_CONFIG_HOME"]
+    home_path = Path(os.environ["XDG_CONFIG_HOME"])
 elif "HOME" in os.environ:
-    home_path = os.environ["HOME"]
+    home_path = Path(os.environ["HOME"])
 elif "APPDATA" in os.environ:
-    home_path = os.environ["APPDATA"]
+    home_path = Path(os.environ["APPDATA"])
 else:
-    home_path = os.path.dirname(__file__)
+    home_path = Path(__file__).parent
 
 # CONFIG #######################################
 
 # WARNING : Hide this file in a safe and secure place
 # If one day you forget your credentials,
 # you'll have to use one of your private keys instead
-PRIVATE_KEY_FILE_PATH = os.path.join(home_path, ".duniter_account_pubsec_v1.duniterkey")
+PRIVATE_KEY_FILE_PATH = Path(home_path, ".duniter_account_pubsec_v1.duniterkey")
 
 ################################################
 

examples/save_and_load_private_key_file_wif.py

 from duniterpy.key import SigningKey
 import getpass
 import os
+from pathlib import Path
 
 if "XDG_CONFIG_HOME" in os.environ:
-    home_path = os.environ["XDG_CONFIG_HOME"]
+    home_path = Path(os.environ["XDG_CONFIG_HOME"])
 elif "HOME" in os.environ:
-    home_path = os.environ["HOME"]
+    home_path = Path(os.environ["HOME"])
 elif "APPDATA" in os.environ:
-    home_path = os.environ["APPDATA"]
+    home_path = Path(os.environ["APPDATA"])
 else:
-    home_path = os.path.dirname(__file__)
+    home_path = Path(__file__).parent
 
 # CONFIG #######################################
 
 # WARNING : Hide this file in a safe and secure place
 # If one day you forget your credentials,
 # you'll have to use one of your private keys instead
-PRIVATE_KEY_FILE_PATH = os.path.join(home_path, ".duniter_account_wif_v1.duniterkey")
+PRIVATE_KEY_FILE_PATH = Path(home_path, ".duniter_account_wif_v1.duniterkey")
 
 ################################################
 

tests/key/test_signing_key.py

-import os
+from pathlib import Path
 
 from duniterpy.key import VerifyingKey, SigningKey, PublicKey
 from duniterpy.key.scrypt_params import ScryptParams
 import unittest
 
-TEST_FILE_PATH = "/tmp/test_file.txt"
+TEST_FILE_PATH = Path("/tmp/test_file.txt")
 
 
 class TestSigningKey(unittest.TestCase):
@@ -12,8 +12,8 @@ class TestSigningKey(unittest.TestCase):
         super(TestSigningKey, self)
 
         # remove test file from disk
-        if os.path.exists(TEST_FILE_PATH):
-            os.unlink(TEST_FILE_PATH)
+        if TEST_FILE_PATH.exists():
+            TEST_FILE_PATH.unlink()
 
     def test_decrypt_seal(self):
         sign_key = SigningKey.from_credentials("alice", "password", ScryptParams())