deprecate RustExtension.py_limited_api

[davidhewitt]

I'd like to simplify the RustExtension options a little further and remove the py_limited_api setting, as I think users should always use "auto" and use [bdist_wheel] configuration to set up an abi3 build.

If the PyO3 module should be built in abi3 mode for local (i.e. non-wheel) builds too, then that configuration is probably better placed in the Cargo.toml than in the setuptools-rust layer.

cc @alex as you implemented this setting originally, but I think cryptography has now also switched to auto mode.

[alex]

Yes, we now use auto + features = ["abi3"] in Cargo.toml + --config-settings=--build-option=--py-limited-api=${{ matrix.PYTHON.ABI_VERSION }} when building release wheels.

Which I think means this is fine for us.

[davidhewitt]

Great! I should probably also add the --config-settings option to the docs too 👍