Show warning when abi3 is used with PyPy

[kngwyu]

Thanks @davidhewitt for contacting to PyPy maintainers.

[davidhewitt]

I think we should definitely include this in the ## Packaging feature at the top of the changelog, because it could break user's builds:

### Packaging
- The `abi3` feature is now a compile error when building for `PyPy`. [#1469](https://github.com/PyO3/pyo3/pull/1469)

On setuptools-rust they'll have to write like this example: https://github.com/PyO3/setuptools-rust/blob/14703b86325e2c53cebec22937e9ca48c7cdeb70/examples/rust_with_cffi/setup.py#L24-L25

[davidhewitt]

Just a couple of wording changes which I think makes it read easier:

Suggested change

	compile_error!(concat!(
	"PyPy still does not support abi3. ",
	"See https://foss.heptapod.net/pypy/pypy/-/issues/3397 for the discussion."
	));
	compile_error!(
	"PyPy does not yet support abi3. \
	See https://foss.heptapod.net/pypy/pypy/-/issues/3397 for more information."
	);

[konstin]

I think it would be good if there was a way to build for pypy when the abi3 feature is enabled; I'd say the abi3 feature should require that only abi3 apis are used, while it should be possible to build non-abi3 wheels. Otherwise someone who wants to ship wheels for both cpython abi3 and pypy would need to add an optional feature for enabling abi3 and make sure cpython wheels are build with the feature and the pypy wheels without it.

[messense]

I think it would be good if there was a way to build for pypy when the abi3 feature is enabled; I'd say the abi3 feature should require that only abi3 apis are used, while it should be possible to build non-abi3 wheels. Otherwise someone who wants to ship wheels for both cpython abi3 and pypy would need to add an optional feature for enabling abi3 and make sure cpython wheels are build with the feature and the pypy wheels without it.

Example: https://github.com/milesgranger/pyrus-cramjam/blob/37306e46ddb288a164af7d65db16eae3e3ce53ff/Makefile#L37

[kngwyu]

Thanks, I changed it to show a warning.
It would look like:

In addition, now this PR includes some refactorings of the build script.

[davidhewitt]

LGTM - just a couple of final suggestions.

The build script refactoring is very nice 👍

[davidhewitt]

👍 thanks!

[konstin]

Thanks, the warning is a good solution 👍