python: difflib prefers unicode strings

Python 3 does not automatically convert from bytes to unicode strings
like Python 2 used to do.

This commit makes sure we pass unicode strings to difflib.unified_diff,
so that the script works on both Python 2 and 3.

Reviewed-by: Dylan Baker <dylan@pnwbakers.com>

src/compiler/glsl/glcpp/tests/glcpp_test.py

@@ -64,8 +64,9 @@ def test_output(glcpp, filename, expfile, nl_format='\n'):
             stderr=subprocess.STDOUT,
             stdin=subprocess.PIPE)
         actual, _ = proc.communicate(f.read())
+        actual = actual.decode('utf-8')
 
-    with open(expfile, 'rb') as f:
+    with open(expfile, 'r') as f:
         expected = f.read()
 
     if actual == expected: