test/py: hush_if_test: Use open() in place of file()

In python 3.x the file() function has been removed. Use open() instead,
which works on both python 2.x & 3.x, and is described as the preferred
method of opening a file by python 2.x documentation anyway.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>