Add Python 3.14 support

[rascani]

Summary

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.

Fixes #16750

This PR was authored with Claude.

Test Plan

• 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

• 📄 Preview Python docs built from this PR

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❌ 2 New Failures, 1 Pending, 1 Unrelated Failure

As of commit 935fef0 with merge base fa49f8f ():

NEW FAILURES - The following jobs have failed:

• pull / unittest / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 2b306e775ee9a1f8779528fbcf76fbe37066a3aed0fd72a0631489b1638d8f80 /exec failed with exit code 1
• pull / unittest-editable / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 693f7c5cb573daab18355c3dc1e6d88779d08312fc79482a927f9f64f13c3a78 /exec failed with exit code 1

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

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

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