add tox docs build to github actions

.github/workflows/tests.yml

@@ -21,6 +21,7 @@ jobs:
         - ["3.9",   "py39"]
         - ["3.10",  "py310"]
         - ["pypy3", "pypy3"]
+        - ["3.10",  "docs"]
 
     runs-on: ubuntu-latest
     name: ${{ matrix.config[1] }}