ensure macos x86_64 wheels get built on non-arm64 GH runner

and allow PyPI uploader to skip existing wheels

.github/workflows/ci.yml

@@ -51,7 +51,9 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [macos-latest, windows-latest]
+        # macos-13 runners are still x86_64, macos-14 (latest) are arm64; we want to build
+        # the x86_64 wheel on/for x86_64 macs
+        os: [macos-13, windows-latest]
         arch: [auto64]
         build: ["*"]
         # temporarily disable building wheels for PyPy until this gets fixed:
@@ -155,3 +157,4 @@ jobs:
       with:
         user: __token__
         password: ${{ secrets.PYPI_PASSWORD }}
+        skip-existing: true