Add generate-abi3-import-lib feature to pyo3-ffi

[messense]

For PyO3/maturin#879

[ravenexp]

LGTM.

I didn't think someone would depend on pyo3-ffi directly.

[messense]

orjson does: https://github.com/ijl/orjson/blob/acaecd2409231106307719f7288e51ebdbb9b24d/Cargo.toml#L74

[messense]

---- test_anext_aiter stdout ----
thread 'test_anext_aiter' panicked at 'called `Result::unwrap()` on an `Err` value: ()', tests/test_proto_methods.rs:779:10
stack backtrace:
   0: rust_begin_unwind
             at /rustc/7737e0b5c4103216d6fd8cf941b7ab9bdbaace7c/library/std/src/panicking.rs:584:5
   1: core::panicking::panic_fmt
             at /rustc/7737e0b5c4103216d6fd8cf941b7ab9bdbaace7c/library/core/src/panicking.rs:143:14
   2: core::result::unwrap_failed
             at /rustc/7737e0b5c4103216d6fd8cf941b7ab9bdbaace7c/library/core/src/result.rs:1749:5
   3: core::result::Result<T,E>::unwrap
             at /rustc/7737e0b5c4103216d6fd8cf941b7ab9bdbaace7c/library/core/src/result.rs:1065:23
   4: test_proto_methods::test_anext_aiter
             at ./tests/test_proto_methods.rs:777:5
   5: test_proto_methods::test_anext_aiter::{{closure}}
             at ./tests/test_proto_methods.rs:750:1
   6: core::ops::function::FnOnce::call_once
             at /rustc/7737e0b5c4103216d6fd8cf941b7ab9bdbaace7c/library/core/src/ops/function.rs:227:5
   7: core::ops::function::FnOnce::call_once
             at /rustc/7737e0b5c4103216d6fd8cf941b7ab9bdbaace7c/library/core/src/ops/function.rs:227:5

There is an unrelated test failure on Python 3.11-dev.

[messense]

pyo3/pyo3-ffi/Cargo.toml

Line 24 in ac56be4

extension-module = []

pyo3/Cargo.toml

Line 70 in ac56be4

extension-module = ["pyo3-build-config/extension-module", "pyo3-ffi/extension-module"]

Should we also add pyo3-build-config/extension-module to pyo3-ffi?