try to fix deprecation 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

tox.ini

@@ -17,7 +17,7 @@ python =
     3.8: 3.8-unit
     3.9: 3.9-unit
     3.10: 3.10-unit
-    pypy3: pypy3-unit
+    pypy-3: pypy3-unit
 
 [testenv]
 sitepackages = False