#[text_signature] doesn't work with abi3

[davidhewitt]

The solution in #1201 for text_signature with abi3 got disabled until Python 3.10, so at the moment I think __text_signature__ will just silently be None if abi3 is enabled.

We might want to consider a more user-friendly solution like failing at compile-time.

Not sure if this blocks 0.13 release - I think it's potentially quite annoying but also I expect no users are using abi3 yet, so it'll only break for them when they switch to abi3.

At the very least we should document the problem alongside abi3 docs before releasing.