whitespace is a substring of smallest leading whitespace.
Based on patch by Robert Li.

Patch by John Mark Vandenberg.

Clarify message and expected action for beginners.
Guard against astral chars in path incorporated in message.

This restores details lost in revision 097f4fda61a4 (since Python 3.3,
related to the new OSError subclasses). Further additions:

* Markup for attributes and constructor signature
* Explain "winerror" and "filename2"
* Extend test to check for filename2 defaulting to None
* Clarify that the constructor can return a subclass

I have intentionally left out any details of allowing more than five
arguments, or how the "args" attribute is set for four or more arguments.
These details seem to be dependent on the Python version and platform.

rather than from elsewhere on PATH.

This removes the need to add externals/tcltk[64]/bin to PATH to be able
to import tkinter.  Also documents the necessity for the DLLs to be
on PATH or in python.exe's directory.

Patch written by Bernt Røskar Brenna.

This should make buildbots green.

Patch by TAKASE Arihiro.

tab in the IDLE Preferences dialog.  Code patch by Mark Roseman.

pickletools.opcodes.

Patch by Serhiy Storchaka.

the other buttons and the new IDLE Dark theme.

Patch by Nir Soffer.

sys.path has an unwritable __pycache__ directory.

This typically comes up when someone runs the test suite from an
administrative install of Python on Windows where the user does not
have write permissions to the stdlib's directory.

Thanks to Zachary Ware and Matthias Klose for reporting bugs related
to this issue.
(grafted from 34bbd537b3e688dfbb6498e9083445a6a72fc4b1)