pygrub: make python scripts work with 2.6 and up

pygrub: make python scripts work with 2.6 and up

Run 2to3 and pick the sensible suggestions.

Import print_function and absolute_import so 2.6 can work.

There has never been a curses.wrapper module according to 2.x and 3.x
doc, only a function, so "import curses.wrapper" is not correct. It
happened to work because 2.x implemented a (undocumented) module.

We only need to import curses to make curses.wrapper available to
pygrub.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>