CI: Replace deprecated pypy3 with pypy-3.8 (#460)

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/test.yml

@@ -9,7 +9,7 @@ jobs:
       fail-fast: false
       max-parallel: 5
       matrix:
-        python-version: ['3.6', '3.7', '3.8', '3.9', '3.10', 'pypy3']
+        python-version: ['3.6', '3.7', '3.8', '3.9', '3.10', 'pypy-3.8']
 
     steps:
     - uses: actions/checkout@v2