List data files are listed in the Distribution attribute 'data_files',

rather than 'data'.

Lib/distutils/command/install_data.py

@@ -17,7 +17,7 @@ class install_data (install_misc):
         self._install_dir_from('install_data')
 
     def run (self):
-        self._copy_files(self.distribution.data)
+        self._copy_files(self.distribution.data_files)
 
     def get_inputs (self):
-        return self.distribution.data or []
+        return self.distribution.data_files or []

Lib/distutils/dist.py

@@ -155,7 +155,7 @@ class Distribution:
         self.include_dirs = None
         self.extra_path = None
         self.scripts = None
-        self.data = None
+        self.data_files = None
 
         # And now initialize bookkeeping stuff that can't be supplied by
         # the caller at all.  'command_obj' maps command names to