python: Use open(), not file()

The latter is a constructor for file objects, but when actually opening
a file, using the former is more idiomatic.

In addition, file() is not a builtin any more in Python 3, so this makes
the script compatible with both Python 2 and Python 3.

Signed-off-by: Mathieu Bridon <bochecha@daitauha.fr>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>