dev(hansbug): use macos11 environment for unittest on macOS

.github/workflows/test.yml

@@ -16,7 +16,7 @@ jobs:
         os:
           - 'ubuntu-20.04'
           - 'windows-2019'  # need to be fixed, see: https://github.com/opendilab/treevalue/issues/41
-          - 'macos-10.15'
+          - 'macos-11'
         python-version:
           - '3.7'
           - '3.8'