Implement function accessors for Python function object

[clin1234]

No description provided.

[davidhewitt]

I think it would be worth having private fn _PyFunction_CAST which does the assert and cast like the 3.15 implementation of these.

[codspeed-hq]

Merging this PR will not alter performance

✅ 141 untouched benchmarks
⏩ 6 skipped benchmarks¹

---

_{Comparing clin1234:patch-13 (97ab000) with main (a30ced8)}

Footnotes

1. 6 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[clin1234]

Build docs on 3.14 ubuntu is failing on this command: cargo doc --lib --no-default-features --features=full --no-deps --workspace --exclude=pyo3-macros --exclude=pyo3-macros-backend:

  error: unreachable call
     --> src/conversions/std/num.rs:179:73
      |
  179 |                     .map_err(|e| exceptions::PyOverflowError::new_err(e.to_string()))
      |                                                                       - ^^^^^^^^^ unreachable call
      |                                                                       |
      |                                                                       any code following this expression is unreachable
  ...
  339 | int_fits_c_long!(i64);
      | --------------------- in this macro invocation
      |
      = note: `-D unreachable-code` implied by `-D warnings`
      = help: to override `-D warnings` add `#[allow(unreachable_code)]`
      = note: this error originates in the macro `int_fits_c_long` (in Nightly builds, run with -Z macro-backtrace for more info)