Use typing_extensions backports so array aliases resolve on Python 3.10/3.11

[nstarman]

Fixes #9.

Problem

The predefined array aliases in bearshape/{jax,numpy,torch,cupy}.py are built with typing.TypeAliasType, added in Python 3.12, and typing.TypeVarTuple, added in 3.11 — but the package declares requires-python = ">=3.10". Any checker resolving 3.10 or 3.11 fails on every subscript of Shaped, F32, IntLike, and friends.

Two more constructs hit the same wall at the declared floor: typing.Self in bearshape/cupy.py (3.11+) and typing.Never in bearshape/_dimensions.py (3.11+). Fixing only TypeAliasType would leave the aliases still broken at 3.10, so this PR covers all four.

The gap is invisible to CI today because [tool.pyright], [tool.mypy], and ty.toml all pin 3.12, and the tox typecheck envs are py313 only.

Change

• Import TypeAliasType, TypeVarTuple, Self, and Never from typing_extensions in the five affected modules.
• Add typing_extensions>=4.6 to dependencies (the version that introduced TypeAliasType) and to uv.lock. It is already an installed transitive dependency in the dev environment.
• CHANGELOG.md entry.

The rest of the diff is ruff format reflowing lines whose length changed with the import style; no other behavior is touched. Runtime behavior is unchanged — every touched name is used only in annotations or under TYPE_CHECKING.

Verification

Before, on main:

pyright --pythonversion	errors
3.10	480
3.12	0

After:

checker	3.10	3.11	3.12	3.13	3.14
pyright	0	0	0	0	0
mypy	clean	—	clean	—	clean
ty	clean	—	clean	—	clean

uv run pytest -n auto → 1064 passed, 5 skipped (CuPy and platform-specific longdouble skips). uv run prek run -a and uv lock --check clean.

Suggested follow-up (not in this PR)

Nothing stops this from regressing, since the checkers are configured at 3.12/3.13 rather than the declared floor. Setting pythonVersion/python_version/python-version to "3.10" in pyproject.toml and ty.toml makes CI check the floor — all three checkers pass at 3.10 with this branch, so it would be a green change today. I left it out because pinning checker targets is a maintainer policy call; happy to add it here or in a separate PR if you want it.

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