bump CI sphinx version 1.6.5 -> 1.8 for tests

Author: rossbar

.github/workflows/test.yml

@@ -13,7 +13,7 @@ jobs:
       matrix:
         os: [Ubuntu]
         python-version: [3.6, 3.7, 3.8, 3.9]
-        sphinx-version: ["sphinx==1.6.5", "sphinx==2.1", "sphinx>3.0"]
+        sphinx-version: ["sphinx==1.8.0", "sphinx==2.1", "sphinx>3.0"]
     steps:
     - uses: actions/checkout@v2