Replace file() with open() for Python 3 compatibility

The built-in named 'file' has been removed since Python 3.0 [1]
This patch replaces it by 'open' which is the same under Python 2 and 3.

[1] https://docs.python.org/release/3.0/whatsnew/3.0.html#builtins

Change-Id: I0d8b6ef1501f7eabc47fb767b0581ac49b646962

test/unit/test_sof_pkg.py

@@ -48,7 +48,7 @@ class TestPkgInfo(unittest.TestCase):
             sc = os.path.join(td, 'saved_config.txt')
             pi.save_config(sc)
             exp = 'NAME=c\nVERSION=a\nRELEASE=b\n'
-            contents = file(sc, 'r').read()
+            contents = open(sc, 'r').read()
             assert contents == exp
         finally:
             shutil.rmtree(td)