Stop testing pypy3.7 which isn't supported by Trio yet

`warn_on_full_buffer` support was added in PyPy, but it did not get
released yet.

Author: pquentin

.github/workflows/ci.yml

@@ -35,7 +35,7 @@ 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', '3.6', '3.7', '3.8', '3.9', '3.6-dev', '3.7-dev', '3.8-dev', '3.9-dev']
         check_formatting: ['0']
         check_docs: ['0']
         extra_name: ['']