macos-12 -> macos-13 (#392)

Author: benjaminp

.github/workflows/ci.yml

@@ -46,11 +46,11 @@ jobs:
             os: "ubuntu-latest"
         include:
           - python-version: "3.5"
-            os: "macos-12"
+            os: "macos-13"
           - python-version: "3.6"
-            os: "macos-12"
+            os: "macos-13"
           - python-version: "3.7"
-            os: "macos-12"
+            os: "macos-13"
           - python-version: "2.7"
             os: "ubuntu-20.04"
           - python-version: "3.5"