Use macOS 11 as a build host for support packages.

macOS 11 was the first release to support Apple Silicon; there are internal checks that look for macOS 11 as a proxy for Apple Silicon support. So - even though you can build Apple Silicon builds on macOS 10.15, it's safer to build on macOS 11 to make sure those checks are enabled. See https://bugs.python.org/issue42619 for details.

.github/workflows/release.yaml

@@ -44,7 +44,7 @@ jobs:
       upload_url: ${{ steps.create_release.outputs.upload_url }}
 
   build:
-    runs-on: macOS-latest
+    runs-on: macOS-11.0
     needs: make-release
     strategy:
       max-parallel: 4