Add Python 3.14 support

[rascani]

Summary

Bump pybind11 from v2.13.6 to v3.0.4, which adds Python 3.14 compatibility. Bridge the legacy PYTHON_EXECUTABLE cmake variable to Python_EXECUTABLE so pybind11 3.0's FindPython module picks up the correct interpreter.

Raise requires-python upper bound from <3.14 to <3.15 and add 3.14 to wheel build matrices and the pull request test matrix.

coremltools has no cp314 wheels yet, so its dependency gets a python_version < '3.14' marker — the CoreML backend is unavailable on 3.14 until Apple ships compatible wheels. scikit-learn is loosened from ==1.7.1 to >=1.7.1 since 1.7.2+ has cp314 wheels.

Bump black from 24.4.2 to 26.3.0 so the linter recognizes py314 as a valid target-version in pyproject.toml.

py::enum_ usage in two pybinding files is deprecated in pybind11 3.0 but still functional; migration to py::native_enum can follow separately.

Fixes #16750

This PR was authored with Claude.

Test Plan

• cmake configure succeeded with pybind11 3.0.4 (--preset pybind)
• Built portable_lib, data_loader, selective_build, _llm_runner pybinding targets — all compiled clean on Python 3.13/macOS
• Verified black 26.3.0 recognizes py314 target-version
• lintrunner passes with updated black
• Full CI validation on Python 3.14 will run via the updated wheel build and pull.yml workflows

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/20452

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❌ You can merge normally! (2 Unrelated Failures), 2 Unclassified Failures

As of commit 7d0b246 with merge base 6021a58 ():

UNCLASSIFIED FAILURES - DrCI could not classify the following jobs because the workflow did not run on the merge base. The failures may be pre-existing on trunk or introduced by this PR:

• Build Windows Wheels / pytorch/executorch / build-wheel-py3_10-cpu (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
• Build Windows Wheels / pytorch/executorch / upload / upload-wheel-py3_10-cpu (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  Unable to download artifact(s): Artifact not found for name: pytorch_executorch__3.10_cpu_x64

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / windows / windows-job (gh) (trunk failure)
  Process completed with exit code 2.
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  Process completed with exit code 2.

This comment was automatically generated by Dr. CI and updates every 15 minutes.

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