fix: apply base constraints to build env

[henryiii]

This hopefully will help with failures like #2542 and main. Currently, we lose all constraints, even our base ones, when there's custom constraints. This adds any required constraints used to generate the constraint file to the constraints always.

[Copilot]

Pull Request Overview

This PR fixes constraint handling in the build environment to prevent losing base constraints when custom constraints are specified. It addresses failures like #2542 by ensuring base constraints are always applied alongside custom ones.

• Changes constraint file generation to use persistent .in files instead of temporary files
• Adds logic to merge custom constraints with base constraints from .in files
• Creates new constraint input files for Pyodide Python versions 3.12 and 3.13

Reviewed Changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
noxfile.py	Updates constraint generation to write to persistent .in files instead of temporary files
cibuildwheel/util/resources.py	Adds CONSTRAINTS_IN constant for the base constraints input file
cibuildwheel/util/packaging.py	Implements logic to merge custom constraints with base constraints from .in files
cibuildwheel/resources/constraints-pyodide313.in	New constraint input file for Python 3.13 Pyodide builds
cibuildwheel/resources/constraints-pyodide312.in	New constraint input file for Python 3.12 Pyodide builds

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