Require Python 3.11 in setup.py

[mattfaltyn]

Summary

• Update the explicit setup.py version guard to require Python 3.11.
• Correct the corresponding error message.

Root cause

Python 3.10 support was removed in af2bd4f, but the direct setup.py guard was not updated alongside pyproject.toml, the installation documentation, and wheel-building configuration.

Impact

Direct setup.py invocations now reject unsupported Python 3.10 consistently with the project metadata and documentation. Supported Python versions are unaffected.

Validation

• Python 3.10: setup.py --name reports Python >= 3.11 is required to build and exits with status 1.
• Python 3.11: setup.py --name reports tensorstore and exits successfully.
• git diff --check
• Bazel: 458 of 459 targets passed in the full sandboxed run with --macos_minimum_os=10.13; the remaining //python/tensorstore:fork_test then passed both tests directly using /private/tmp after the sandbox encountered a macOS privacy denial on its default temporary directory.

Fixes #292

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