CI: Replace deprecated pypy3 with pypy-3.8 (#1533)

pypy3 is deprecated and is not available in newer images:
actions/setup-python#244 (comment)

Instead explicitly specify the version:
https://github.com/actions/setup-python#specifying-a-pypy-version

Committed via https://github.com/asottile/all-repos

.github/workflows/cron.yml

@@ -75,7 +75,7 @@ jobs:
           - MacOS
           - Windows
         python-version:
-          - pypy3
+          - pypy-3.8
         pip-version:
           - latest
     env: