Use 'Linux' (dark bg) colours for Windows

This puts the colours in inspect output and tracebacks back to what
they were in IPython 4.x, but leaves the prompt colouring as in 5.0.
I tried changing the colour scheme to 'Linux' entirely for Windows,
but that selects Monokai as the theme for prompt_toolkit, which looks
pretty horrible in 16 colours (at least to my eyes).

This is admittedly a hack, but hopefully our legacy colour system is
on the way out anyway.

Closes gh-9723

IPython/utils/PyColorize.py

@@ -146,7 +146,15 b' NeutralColors = ColorScheme('
     'normal'         : Colors.Normal  # color off (usu. Colors.Normal)
     }  )
 
-
+# Hack: the 'neutral' colours are not very visible on a dark background on
+# Windows. Since Windows command prompts have a dark background by default, and
+# relatively few users are likely to alter that, we will use the 'Linux' colours,
+# designed for a dark background, as the default on Windows. Changing it here
+# avoids affecting the prompt colours rendered by prompt_toolkit, where the
+# neutral defaults do work OK.
+
+if os.name == 'nt':
+    NeutralColors = LinuxColors.copy(name='Neutral')
 
 LightBGColors = ColorScheme(
     'LightBG',{