Restore Python 2.1 compatibility (os.extsep was introduced in Python 2.2).
diff --git a/Lib/distutils/command/install_lib.py b/Lib/distutils/command/install_lib.py
index 22d0ab3..08ff543 100644
--- a/Lib/distutils/command/install_lib.py
+++ b/Lib/distutils/command/install_lib.py
@@ -9,8 +9,10 @@
 
 
 # Extension for Python source files.
-PYTHON_SOURCE_EXTENSION = os.extsep + "py"
-
+if hasattr(os, 'extsep'):
+    PYTHON_SOURCE_EXTENSION = os.extsep + "py"
+else:
+    PYTHON_SOURCE_EXTENSION = ".py"
 
 class install_lib (Command):