test: tox-gh-actions says to do it like this

```
Warning: WARNING: PendingDeprecationWarning
Support of old-style PyPy config keys will be removed in tox-gh-actions v3.
Please use "pypy-2" and "pypy-3" instead of "pypy2" and "pypy3".

Example of tox.ini:
[gh-actions]
python =
    pypy-2: pypy2
    pypy-3: pypy3
    # The followings won't work with tox-gh-actions v3
    # pypy2: pypy2
    # pypy3: pypy3
```

Author: nedbat

tox.ini

@@ -100,4 +100,4 @@ python =
     3.10: py310
     3.11: py311
     3.12: py312
-    pypy3: pypy3
+    pypy-3: pypy3