Use the presence of builtins rather than configparser to indicate the future/src folder

Author: edschofield

setup.py

@@ -135,7 +135,9 @@
                           '_markupbase',
                           '_thread',
                           'builtins',
-                          # 'configparser',
+                          # Catch the case that configparser is in the build folder
+                          # from a previous version of `future`:
+                          'configparser',
                           'copyreg',
                           'html',
                           'http',

src/future/standard_library/__init__.py

@@ -755,7 +755,9 @@ def __enter__(self):
         self.old_sys_modules = copy.copy(sys.modules)
         if sys.version_info[0] < 3:
             return
-        FUTURE_SOURCE_SUBFOLDERS = ['future', 'past', 'libfuturize', 'configparser']
+        # The presence of all these indicates we've found our source folder,
+        # because `builtins` won't have been installed in site-packages by setup.py:
+        FUTURE_SOURCE_SUBFOLDERS = ['future', 'past', 'libfuturize', 'libpasteurize', 'builtins']
 
         # Look for the future source folder:
         for folder in self.old_sys_path: