fix: compile with PyPy when nightly feature is enabled

[chirizxc]

From lava-sh/yaml-rs CI

[21](https://github.com/lava-sh/yaml-rs/actions/runs/27904060735/job/82569415590?pr=146#step:7:122)
   Compiling pyo3-macros v0.29.0
error[E0425]: cannot find type `PyWeakReference` in module `crate::ffi`
   --> C:\Users\runneradmin\.cargo\registry\src\index.crates.io-1949cf8c6b5b557f\pyo3-0.29.0\src\marker.rs:289:37
    |
289 |         impl !Ungil for crate::ffi::PyWeakReference {}
    |                                     ^^^^^^^^^^^^^^^ not found in `crate::ffi`

error[E0425]: cannot find type `PyDictKeysObject` in module `crate::ffi`
   --> C:\Users\runneradmin\.cargo\registry\src\index.crates.io-1949cf8c6b5b557f\pyo3-0.29.0\src\marker.rs:293:37
    |
293 |         impl !Ungil for crate::ffi::PyDictKeysObject {}
    |                                     ^^^^^^^^^^^^^^^^
    |
   ::: C:\Users\runneradmin\.cargo\registry\src\index.crates.io-1949cf8c6b5b557f\pyo3-ffi-0.29.0\src\cpython\dictobject.rs:42:1
    |
 42 | pub struct PyDictObject {
    | ----------------------- similarly named struct `PyDictObject` defined here
    |
help: a struct with a similar name exists
    |
293 -         impl !Ungil for crate::ffi::PyDictKeysObject {}
293 +         impl !Ungil for crate::ffi::PyDictObject {}
    |

For more information about this error, try `rustc --explain E0425`.
error: could not compile `pyo3` (lib) due to 2 previous errors
💥 maturin failed
  Caused by: Failed to build a native library through cargo
  Caused by: Cargo build finished with "exit code: 101": `"cargo" "rustc" "--features" "mimalloc" "--message-format" "json-render-diagnostics" "--manifest-path" "D:\\a\\yaml-rs\\yaml-rs\\Cargo.toml" "--lib"`
Error: Process completed with exit code 1.

[davidhewitt]

Thanks! Can you please add a test for this, probably by adding a check-nightly job which runs uvx nox -s check-all but is otherwise the same as the existing clippy jobs (in ci.yml)

(I confirmed that locally running RUSTUP_TOOLCHAIN=nightly uvx nox -s check-all is already good enough to reproduce the issue.)

[davidhewitt]

Thanks, will tweak one line and send to merge