python: bump to 2.7.6

Even though jumping from 2.7.3 to 2.7.6 looks like a minor version
bump, it is in fact a fairly significant one, because a good number of
changes to help cross-compilation have been merged into Python
upstream. Therefore, most of our patches are affected by this change.

In detail, this commit:

 * Renames all the patches to follow the naming convention of patches
   in Buildroot: the patch file names should not have any version
   number.

 * The patches numbered above 100, that add configuration options to
   disable certain modules of the Python standard library, are only
   renamed and slightly adapted, they didn't change that much.

 * The patches numbered below 100 are almost entirely rewritten: many
   of the cross-compilation problems that used to exist in Python
   2.7.3 no longer exist, and the number of remaining problems is
   smaller, and can be fixed with smaller patches.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>