Fix overlapping slots, base classes without slots

Fix overlapping slots, base classes without slots

Some `__slots__` were not in order.

Fixes #7527

### Description

I'm fixing two types of slots mistakes:
- [x] remove overlapping slots (i.e. slots already defined on a base class)
- [x] fix broken inheritance (i.e. slots class inheriting from a non-slots class)
  - [x] slots added to base class `TransactionalContext`. It seemed to use two attributes, which I've added as slots.
  - [x] empty slots removed from `ORMOption`. Its base class explicitly makes use of `__dict__` so empty slots don't add anything.
  - [x] empty slots added to `PostLoader`. It doesn't appear to use any slots not already defined on its base classes.
  - [x] empty slots added to `IterateMappersMixin`. It doesn't appear to use any slots not already defined on its subclasses.
  - [x] empty slots added to `ImmutableContainer`. It doesn't use any fields.
  - [x] empty slots added to `OperatorType`. It's a protocol.
  - [x] empty slots added to `InternalTraversal`, `_HasTraversalDispatch`. They don't seem to use attributes on their own.

### Checklist

This pull request is:

- [x] A short code fix
	- please include the issue number, and create an issue if none exists, which
	  must include a complete example of the issue.  one line code fixes without an
	  issue and demonstration will not be accepted.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.   one line code fixes without tests will not be accepted.

**Have a nice day!**

Closes: #7589
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/7589
Pull-request-sha: 70a9c4d46916b7c6907eb1d3ad4f7033ec964191

Change-Id: I6c6e3e69c3c34d0f3bdda7f0684849834fdd1863