Support slicing header sequences

[xiehuanyi]

The Headers sequence exposed through Request.headers and other events currently treats slice indices as integer indices. A one-element or empty slice raises ValueError; a three-element slice returns internal three-tuples rather than the public (name, value) pairs.

Handle slices by returning a new Headers instance with the selected entries. This keeps normalized byte-pair iteration and original header-name casing available through raw_items(). Integer indexing and wire parsing retain their existing behavior.

Validation on Python 3.12.3: five regression cases fail against upstream (ordinary, exactly three elements, empty, reverse and stride slices); all 83 project tests pass with the fix. Black, isort and mypy checks pass. The changed header module has 100% coverage. Added a news fragment, to be renamed to this PR's number immediately after creation as described in the contributing guide.

Prepared with AI assistance and independently reviewed before submission.

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