Manage append and replace User modify.

src/phi/async_ldap/model.py

@@ -272,10 +272,16 @@ class User(Hackers):
             if not k == "dn":
                 self._entry[k] = v
 
-    async def modify(self, key, value):
+    async def modify(self, key, value, append=False):
         async with self.client.connect(is_async=True) as conn:
             self._entry.connection = conn
-            await self._entry.change_attribute(key, LDAPModOp.REPLACE, value)
+            try:
+                if not append:
+                    del self._entry[key]
+                self._entry[key] = value
+            except KeyError:
+                raise PhiAttributeMissing
+            await self._entry.modify()
 
     async def remove(self):
         async with self.client.connect(is_async=True) as conn:
@@ -291,6 +297,10 @@ class PhiUserDoesNotExist(Exception):
     pass
 
 
+class PhiAttributeMissing(Exception):
+    pass
+
+
 class Service(Robots):
     """
     This class models a system user (i.e. users that are ancillary to