3.6 and 3.7 are no longer available on deadsnakes/nightly

Author: graingert

.github/workflows/ci.yml

@@ -59,7 +59,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python: ['pypy-3.6', 'pypy-3.7', '3.6', '3.7', '3.8', '3.9', '3.6-dev', '3.7-dev', '3.8-dev', '3.9-dev']
+        python: ['pypy-3.6', 'pypy-3.7', '3.6', '3.7', '3.8', '3.9', '3.8-dev', '3.9-dev']
         check_formatting: ['0']
         pypy_nightly_branch: ['']
         extra_name: ['']