Issue #25488: merge idle.py from 3.5.

Lib/idlelib/idle.py

 import os.path
 import sys
 
-# If we are working on a development version of IDLE, we need to prepend the
-# parent of this idlelib dir to sys.path.  Otherwise, importing idlelib gets
-# the version installed with the Python used to call this module:
+
+# Enable running IDLE with idlelib in a non-standard location.
+# This was once used to run development versions of IDLE.
+# Because PEP 434 declared idle.py a public interface,
+# removal should require deprecation.
 idlelib_dir = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
-sys.path.insert(0, idlelib_dir)
+if idlelib_dir not in sys.path:
+    sys.path.insert(0, idlelib_dir)
 
-import idlelib.pyshell
-idlelib.pyshell.main()
+from idlelib.pyshell import main  # This is subject to change
+main()