#11985: update docstring of platform.python_implementation.

Author: ezio-melotti

Lib/platform.py

@@ -1398,9 +1398,10 @@ def python_implementation():
     """ Returns a string identifying the Python implementation.
 
         Currently, the following implementations are identified:
-        'CPython' (C implementation of Python),
-        'IronPython' (.NET implementation of Python),
-        'Jython' (Java implementation of Python).
+          'CPython' (C implementation of Python),
+          'IronPython' (.NET implementation of Python),
+          'Jython' (Java implementation of Python),
+          'PyPy' (Python implementation of Python).
 
     """
     return _sys_version()[0]