Revert "refactor: use class attribute"

This reverts commit 125fdfb.

knox/auth.py

@@ -27,11 +27,9 @@ class TokenAuthentication(BaseAuthentication):
     - `request.auth` will be an `AuthToken` instance
     '''
 
-    auth_header_prefix = knox_settings.AUTH_HEADER_PREFIX
-
     def authenticate(self, request):
         auth = get_authorization_header(request).split()
-        prefix = self.auth_header_prefix.encode()
+        prefix = self.authenticate_header(request).encode()
 
         if not auth:
             return None
@@ -89,7 +87,7 @@ def validate_user(self, auth_token):
         return (auth_token.user, auth_token)
 
     def authenticate_header(self, request):
-        return self.auth_header_prefix
+        return knox_settings.AUTH_HEADER_PREFIX
 
     def _cleanup_token(self, auth_token) -> bool:
         for other_token in auth_token.user.auth_token_set.all():