pyo3-build-config: Build "abi3" extensions without an interpreter

[ravenexp]

Ignore the build host Python interpreter configuration when
compiling portable "abi3" extension modules.
Such extensions should not depend on a concrete Python version
or build configuration by design.

Maturin already does this when building "abi3" extension wheels
by defining PYO3_NO_PYTHON=1 for Cargo when an abi3-py3*
feature is detected.

Closes #2292

[davidhewitt]

The only concern is cases like cryptography, where I think they build using abi3-py36 yet also need to query the interpreter to get SSL information etc. - see pyca/cryptography#6695 (comment) and #2092

[ravenexp]

The only concern is cases like cryptography, where I think they build using abi3-py36 yet also need to query the interpreter to get SSL information etc. - see pyca/cryptography#6695 (comment) and #2092

Oh, I can now see how this will break things like https://github.com/aganders3/hello-pyo3-build-config/blob/main/build.rs when an abi3-py3* feature is enabled.

I'll try a different approach then: making PyO3 not fail the build when the default Python interpreter is not found or too old or otherwise unusable, but the extension module can still be built without it. It will at least prevent unnecessary build failures like this: PyO3/maturin#875 (comment).

[davidhewitt]

👍 this new approach looks good to me, thanks!