[gyb] Popen explicit string instead of byte sequence

The Popen command on Python returns a byte sequence on stdout by
default. However by sending the constructor the argument
`universal_newlines=True` it forces the Popen to put a string on stdout.

This was not a problem on Python 2 because the Python 2 regex engine
seemed to work find on byte sequences where Python 3's does not. By
explicitly converting everything to a string the same behavior is now
seen on Python 2 and 3.

See: https://docs.python.org/2/library/subprocess.html#frequently-used-arguments
See: https://docs.python.org/3/library/subprocess.html#frequently-used-arguments

Author: RLovelett

utils/line-directive

@@ -71,7 +71,10 @@ def run():
         sources = sys.argv[1:dashes]
 
         command = subprocess.Popen(
-            sys.argv[dashes + 1:], stderr = subprocess.STDOUT, stdout = subprocess.PIPE
+            sys.argv[dashes + 1:],
+            stderr = subprocess.STDOUT,
+            stdout = subprocess.PIPE,
+            universal_newlines = True
         )
 
         error_pattern = re.compile(