Allow building for non-abi3 pypy wheels when the abi3 feature is enabled

[konstin]

Currently, activating the abi3 feature means that you can't build wheels for pypy, as abi3 wheel are incompatible with pypy and the abi3 causes -i to be ignored. There should be a solution that both builds for abi3 by defaults and allows building for pypy if requested.

[kngwyu]

But doesn't it require upstream support? https://foss.heptapod.net/pypy/pypy/-/issues/3397

[davidhewitt]

FYI for PyO3 0.14 we're going to make abi3 + PyPy a hard error: PyO3/pyo3#1469

[konstin]

Sorry, I misphrased that: It should be possible to build a non-abi3 pypy wheels even when the abi3 feature is enabled.

Once pypy also has a stable abi, there should be switch to build for both, but that's something to figure out once we know the details about pypy's abi3