codegen: Treat input file as binary

Under C locale, open() in Python 3 sets the file encoding to ASCII.
As expat looks at encoding="..." in XML declaration, gdbus-codegen can
simply open the input file as binary and let expat decode the content.

https://bugzilla.gnome.org/show_bug.cgi?id=696633

gio/gdbus-2.0/codegen/codegen_main.py

@@ -166,7 +166,7 @@ def codegen_main():
 
     all_ifaces = []
     for fname in args:
-        f = open(fname)
+        f = open(fname, 'rb')
         xml_data = f.read()
         f.close()
         parsed_ifaces = parser.parse_dbus_xml(xml_data)