Although `@property` is the most common case, any descriptors are now
properly supported.
The special casing of methods goes away as they work by having functions
implement the descriptor protocol. (`__get__` has some special behavior
to make this possible.)
This is some of the most cursed code I have ever written, yet it is
obviously necessary.
On Python <3.10, classes without annotations do not get an
`__annotations__` member at all, so the `getattr` on a subclass falls
back to the parent class `__annotations__`, attempting to create
signature members twice. Fix that by looking at the `__dict__` instead.
