Codex/python 315 cffi compat

[arpitkjaiswal]

Summary

• Require CFFI 2.1.1 or newer on Python 3.15 for both runtime and build dependencies.
• Keep the build requirements input in sync and document the change.

CFFI 2.1.1 avoids an ABI issue affecting Python 3.15.

Validation

• Retried the manylinux_2_31_armv7l job; it passed.
• The initial failure happened during runner setup, when its Docker container stopped.
• nox -e local was not run because the repository was not checked out locally.