mercurial: don't load C extensions from PyPy

PyPy isn't compatible with Python C extensions.

With this patch, the module load policy is automatically to "Python
only" when run under PyPy. `hg` and other Python scripts importing
mercurial.* modules will run from the source checkout or any
installation when executed with PyPy. This should enable people to
more easily experiment with PyPy and its potentially significant
performance benefits over CPython!

mercurial/__init__.py

@@ -24,6 +24,13 b" modulepolicy = '@MODULELOADPOLICY@'"
 if modulepolicy == '@' 'MODULELOADPOLICY' '@':
     modulepolicy = 'c'
 
+# PyPy doesn't load C extensions.
+#
+# The canonical way to do this is to test platform.python_implementation().
+# But we don't import platform and don't bloat for it here.
+if '__pypy__' in sys.builtin_module_names:
+    modulepolicy = 'py'
+
 # Environment variable can always force settings.
 modulepolicy = os.environ.get('HGMODULEPOLICY', modulepolicy)