github CI: test on py310 (again)

.github/workflows/ci.yml

@@ -66,9 +66,9 @@ jobs:
             - os: ubuntu-20.04
               python-version: '3.9'
               toxenv: py39
-            #- os: ubuntu-20.04
-            #  python-version: '3.10-dev'
-            #  toxenv: py310
+            - os: ubuntu-20.04
+              python-version: '3.10-dev'
+              toxenv: py310
             - os: macos-latest
               # note: it seems that 3.8 and 3.9 are currently broken,
               # neverending RuntimeError crashes...