Replace cached_property on slotted classes with a new descriptor instead of writing __getattr__

[DavidCEllis]

Summary

This PR reimplements cached_property in slotted classes as a slot wrapping descriptor replacing the current generated __getattr__ implementation.

Accessing the attributes will probably be slightly slower due to going through this descriptor but if the performance is reasonable this will close #1288 #1325 and #1333.

This idea came up again as part of discussion on https://discuss.python.org/t/extensible-cached-property/105188/25. I hadn't realised it would work, but it looks like there was a partial attempt to implement it for attrs before in #1357

I've implemented it for my own attrs/dataclasses-like here (the _SlottedCachedProperty code is mostly identical).

As I saw there were outstanding issues for it and the code is fairly similar I thought you might also find it useful. (Passing all of your existing tests also gave me some confidence that it does work correctly).

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Changes:

• Adds a _SlottedCachedProperty descriptor class that replaces the slot descriptors after attrs has rebuilt the class.
  • This is very similar to cached_property itself, other than reading/writing/deleting a wrapped slot rather than dict entry.
• Removes the customised __getattr__ function you were making previously.
  • There are probably some redundant tests that were checking the __getattr__ behaviour that are now just checking __getattr__ works in Python.

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Outstanding issues:

This was already the case for the __getattr__ implementation but I'm now aware of one difference in behaviour this hasn't yet resolved as it would require changing the logic of when you create a new cached property. There's currently a test marked as xfail for this.

import attrs
import functools

for slotted in [False, True]:
  @attrs.define(slots=slotted)
  class Parent:
      name: str = "Alice"
  
  @attrs.define(slots=slotted)
  class Child(Parent):
      @functools.cached_property
      def name(self):
          return "Bob"
  
  # This isn't to imply that this is good
  # just that it's consistent
  p = Parent()
  c = Child()
  
  assert p.name == "Alice"
  assert c.name == "Alice"
  del c.name
  assert c.name == "Bob"  # True without slots, errors under slots.

This is because the cached_property replacement check attrs does skips field names that already exist, so name is skipped in the check and doesn't get replaced. The descriptor would behave like this, but it's never placed.

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