fixup: use quotes for py versions

otherwise github action badly parses 3.10 and tries to install python 3.1.

.github/workflows/ci.yml

@@ -52,27 +52,27 @@ jobs:
       matrix:
         include:
             - os: ubuntu-20.04
-              python-version: 3.5
+              python-version: '3.5'
               toxenv: py35
             - os: ubuntu-20.04
-              python-version: 3.6
+              python-version: '3.6'
               toxenv: py36
             - os: ubuntu-20.04
-              python-version: 3.7
+              python-version: '3.7'
               toxenv: py37
             - os: ubuntu-20.04
-              python-version: 3.8
+              python-version: '3.8'
               toxenv: py38
             - os: ubuntu-20.04
-              python-version: 3.9
+              python-version: '3.9'
               toxenv: py39
             - os: ubuntu-20.04
-              python-version: 3.10
+              python-version: '3.10'
               toxenv: py310
             - os: macos-latest
               # note: it seems that 3.8 and 3.9 are currently broken,
               # neverending RuntimeError crashes...
-              python-version: 3.7
+              python-version: '3.7'
               toxenv: py37
 
     env: