Use Django's force_bytes() instead of bytes()

Unlike bytes() which accepts only a string, force_bytes() accept all
sort of input type. This is required since SECRET_KEY may be either a
string or bytes.

pwdb/models.py

@@ -3,6 +3,7 @@ from cryptography.hazmat.primitives.kdf.hkdf import HKDF
 from cryptography.hazmat.primitives import hashes, padding
 from cryptography.hazmat.backends import default_backend
 from django.utils.translation import ugettext_lazy as _
+from django.utils.encoding import force_bytes
 from django.conf import settings
 from django.db import models
 from neluser.models import NelUser
@@ -34,7 +35,7 @@ class SharedPassword(models.Model):
             info=None,
         )
         key = key or settings.SECRET_KEY
-        key = bytes(key, encoding=ENCODING)
+        key = force_bytes(key, encoding=ENCODING)
         return hkdf.derive(key)
 
     @staticmethod
@@ -44,7 +45,7 @@ class SharedPassword(models.Model):
 
     @staticmethod
     def padd_password(clear_password):
-        clear_password = bytes(clear_password, encoding=ENCODING)
+        clear_password = force_bytes(clear_password, encoding=ENCODING)
         padder = padding.PKCS7(BLOCK_SIZE).padder()
         padded_password = padder.update(clear_password) + padder.finalize()
         return padded_password