Remove Cpython 3.6-dev and 3.7-dev from CI (no longer available)

Author: touilleMan

.github/workflows/ci.yml

@@ -35,7 +35,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python: ['pypy-3.6', '3.6', '3.7', '3.8', '3.9', '3.6-dev', '3.7-dev', '3.8-dev', '3.9-dev']
+        python: ['pypy-3.6', '3.6', '3.7', '3.8', '3.9', '3.8-dev', '3.9-dev']
         check_formatting: ['0']
         check_docs: ['0']
         extra_name: ['']