Update pyo3.yml to use macos-15-intel instead of macos-13 (#332)

GitHub is retiring the macos-13 image for GH-hosted runners.

Author: pmaslana

.github/workflows/pyo3.yml

@@ -126,7 +126,7 @@ jobs:
     strategy:
       matrix:
         platform:
-          - runner: macos-13
+          - runner: macos-15-intel
             target: x86_64
           - runner: macos-14
             target: aarch64