Tests(GHActions): Exclude Python 3.8 on OSX

Author: phdru

.github/workflows/run-tests.yaml

@@ -20,6 +20,8 @@ jobs:
           python-version: "3.6"
         - os: macos-latest
           python-version: "3.7"
+        - os: macos-latest
+          python-version: "3.8"
         include:
         - os: ubuntu-latest
           os-name: Linux