fixup: use 3.10-dev as it is not released yet

.github/workflows/ci.yml

@@ -67,7 +67,7 @@ jobs:
               python-version: '3.9'
               toxenv: py39
             - os: ubuntu-20.04
-              python-version: '3.10'
+              python-version: '3.10-dev'
               toxenv: py310
             - os: macos-latest
               # note: it seems that 3.8 and 3.9 are currently broken,