Add tests on macOS 11 in CI

Author: flashcode

.github/workflows/ci.yml

@@ -12,7 +12,8 @@ jobs:
       matrix:
         os:
           - ubuntu-20.04
-          - macos-latest
+          - macos-11
+          - macos-10.15
         python-version:
           - "3.6"
           - "3.7"