Build x64 macOS wheels for 10.13 build target (#716)

Author: joeyballentine

.github/workflows/build_wheels_macos.yml

@@ -49,7 +49,7 @@ jobs:
       SDIST: ${{ matrix.build_sdist || 0 }}
       ENABLE_HEADLESS: ${{ matrix.without_gui }}
       ENABLE_CONTRIB: ${{ matrix.with_contrib }}
-      MACOSX_DEPLOYMENT_TARGET: 10.15
+      MACOSX_DEPLOYMENT_TARGET: 10.13
     steps:
     - name: Cleanup
       run: find . -mindepth 1 -delete