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]

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❗ 1 Active SEVs

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• [ROCm] MI350 CI jobs will have longer queue times due to CI migration

❌ 6 New Failures, 2 Unrelated Failures, 4 Unclassified Failures

As of commit c6f4bf3 with merge base 1b726b2 ():

NEW FAILURES - The following jobs have failed:

• pull / test-arm-backend-no-driver (test_pytest_ops_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t 8036608d2a08f59625bf02ac47f1605fb72a43d738aeff9938cb2fcca363aaeb /exec failed with exit code 1
• pull / unittest / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 0e7a8465c269c247978ae1ca9c16d15337b1a8eec1f54ae8faa0f3e51e732770 /exec failed with exit code 1
• pull / unittest / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 139
• pull / unittest-editable / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 99756ba9db99bd7cb79203af25e35bef3fafaa58f11d6c98dbe51644b5fcbd8a /exec failed with exit code 1
• pull / unittest-editable / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 139
• Test QNN Backend / test-qnn / test-backend-linux (qnn, models) / linux-job (gh)
  RuntimeError: Command docker exec -t 6fa13e0adbcf878f3528652dc79be57be9b0a3e5ecea757046dd3fb153d960ba /exec failed with exit code 92

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 macOS 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)
  Process completed with exit code 134.
• Build macOS 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_
• pull / test-coreml-bc-macos (macos-m2-stable) / macos-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 134
• pull / test-qnn-wheel-packages-linux (3.14) / linux-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  AttributeError: 'typing.Union' object has no attribute '__module__' and no __dict__ for setting new attributes. Did you mean: '__reduce__'?

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-coreml-bc-macos (macos-m1-stable) / macos-job (gh) (detected as infra flaky with no log or failing log classifier)

BROKEN TRUNK - The following job failed but was present on the merge base:

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

• pull / test-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / linux-job (gh) (trunk failure)

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