ci: always use constraints.txt

Let's ensure our setuptools constraint is honored everywhere.
Otherwise we get spammed with deprecation warnings and have to
reason about inconsistent behaviors.

Author: indygreg

.github/workflows/wheel.yml

@@ -38,7 +38,7 @@ jobs:
 
       - name: Build Wheel
         run: |
-          docker run -e PYPATH=/opt/python/${{ matrix.py }} -e ZSTD_WARNINGS_AS_ERRORS=1 --rm -v `pwd`:/project quay.io/pypa/${{ matrix.image }} /project/ci/build-manylinux-wheel.sh
+          docker run -e PIP_CONSTRAINT=/project/ci/constraints.txt -e PYPATH=/opt/python/${{ matrix.py }} -e ZSTD_WARNINGS_AS_ERRORS=1 --rm -v `pwd`:/project quay.io/pypa/${{ matrix.image }} /project/ci/build-manylinux-wheel.sh
 
       - name: Upload Wheel
         uses: actions/upload-artifact@v4
@@ -65,6 +65,7 @@ jobs:
       CIBW_BUILD: ${{ matrix.py }}-*
       CIBW_BUILD_VERBOSITY: '1'
       CIBW_PRERELEASE_PYTHONS: '1'
+      PIP_CONSTRAINT: ci/constraints.txt
       ZSTD_WARNINGS_AS_ERRORS: '1'
     steps:
       - name: Set up Python
@@ -111,6 +112,7 @@ jobs:
             arch: 'arm64'
     runs-on: ${{ matrix.arch == 'arm64' && 'windows-11-arm' || 'windows-2022' }}
     env:
+      PIP_CONSTRAINT: ci/constraints.txt
       ZSTD_WARNINGS_AS_ERRORS: '1'
     steps:
       - name: Set up Python