use PyPy3.9 explicitely in GitHub actions

.github/workflows/tests.yml

@@ -17,7 +17,7 @@ jobs:
         - ["3.8",   "py38"]
         - ["3.9",   "py39"]
         - ["3.10",  "py310"]
-        - ["pypy3", "pypy3"]
+        - ["pypy3.9", "pypy3"]
         - ["3.10",  "docs"]
 
     runs-on: ubuntu-latest