ci: honor cython pin under build[uv] with UV_CONSTRAINT

[bdraco]

What do these changes do?

Add UV_CONSTRAINT and UV_BUILD_CONSTRAINT to the
[tool.cibuildwheel.environment] block, mirroring the existing
PIP_CONSTRAINT and PIP_BUILD_CONSTRAINT entries; both point at
requirements/cython.txt.

Since the cibuildwheel build frontend defaults to build[uv] here,
the PIP_CONSTRAINT env var is silently ignored during the wheel
build; uv pip reads only UV_CONSTRAINT and
UV_BUILD_CONSTRAINT. The PIP_ variants are kept because the
odd-arch matrix in ci-cd.yml overrides
CIBW_BUILD_FRONTEND=build (pip frontend) on containers that do
not ship uv.

Are there changes in behavior for the user?

No user-facing change; the Cython version pin used at wheel build
time is now actually enforced under the build[uv] frontend,
matching the intent of the existing PIP_CONSTRAINT line.

Related issue number

Port of aio-libs/yarl#1729.

Checklist

• I think the code is well written
• Unit tests for the changes exist (N/A, CI/packaging env change)
• Documentation reflects the changes (N/A)
• If you provide code modifications, please add yourself to CONTRIBUTORS.txt
• Add a new news fragment into the CHANGES folder