CI: drop hardcoded architecture: x64 for setup-python

On macos-latest (arm64), the x64 Python 3.10.20 image used by
actions/setup-python currently fails to start with "Library not loaded:
/usr/local/opt/gettext/lib/libintl.8.dylib". Dropping the architecture
constraint lets setup-python pick the native arch (arm64 on
macos-latest, x64 on ubuntu/windows), which is what we actually want
post-3.9 — x64 was only needed for Python 3.9 on Intel macOS. Ubuntu
and Windows already run native x64 regardless.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: adrian-prior

.github/workflows/pull_request.yml

@@ -63,7 +63,6 @@ jobs:
         uses: actions/setup-python@a309ff8b426b58ec0e2a45f0f869d46889d02405 # v6.2.0
         with:
           python-version: ${{ matrix.python-version }}
-          architecture: x64
 
       - name: Install uv
         uses: astral-sh/setup-uv@08807647e7069bb48b6ef5acd8ec9567f424441b # v8.1.0