doc: fix compatibility with ipython 7.11+

[sandrotosi]

Hello,
since we introduced ipython 7.11 in Debian cycler failed to build its documentation, the fix is pretty straightforward:

--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -48,8 +48,6 @@ hashable (as it will eventually be used
 
    from __future__ import print_function
    from cycler import cycler
-
-
    color_cycle = cycler(color=['r', 'g', 'b'])
    color_cycle

can you please apply it? thanks!