Handle all entity types on isinstance checks

Only the uses of `isinstance` against `InputPeer*` types were
reviewed. Notably, `utils` is exempt on this because it needs
to deal with everything on a case-by-case basis.

Since the addition of `*FromMessage` peers, any manual `isinstance`
checks to determine the type were prone to breaking or being
forgotten to be updated, so a common `helpers._entity_type()`
method was made to share this logic.

Since the conversion to `Peer` would be too expensive, a simpler
check against the name is made, which should be fast and cheap.

telethon/client/dialogs.py

@@ -3,7 +3,7 @@ import inspect
 import itertools
 import typing
 
-from .. import utils, hints
+from .. import helpers, utils, hints
 from ..requestiter import RequestIter
 from ..tl import types, functions, custom
 
@@ -436,10 +436,11 @@ class DialogMethods:
                 await client.delete_dialog('username')
         """
         entity = await self.get_input_entity(entity)
-        if isinstance(entity, types.InputPeerChannel):
+        ty = helpers._entity_type(entity)
+        if ty == helpers._EntityType.CHANNEL:
             return await self(functions.channels.LeaveChannelRequest(entity))
 
-        if isinstance(entity, types.InputPeerChat):
+        if ty == helpers._EntityType.CHAT:
             result = await self(functions.messages.DeleteChatUserRequest(
                 entity.chat_id, types.InputUserSelf()))
         else: