Describe the workflow you want to enable

Upper bound the build dependencies on release branches makes it easier to build the wheel in the future. This has two benefits:

• The wheels become easier to build when using the newest build dependency does not work. (Historically, I've seen issues with Cython)
• If we wanted to backport a fix the wheel building is more stable.

Describe your proposed solution

On release branches, provide a upper bound to the build dependencies in pyproject.toml.

SciPy does this already: https://github.com/scipy/scipy/blob/e3228cdfe42e403ed203db16e4db4822eb416797/pyproject.toml#L1-L13

Describe alternatives you've considered, if relevant

Leave the build dependencies to be unbounded.