build: always use utf-8 when reading/writing text files

open() in text mode should never be used without an encoding because it defaults
to the locale encoding which is rarely what is wanted.

This fixes the Windows build in some cases (depending on the locale/paths used)

gtk/gen-gtk-gresources-xml.py

@@ -82,7 +82,7 @@ xml += '''
 
 if len(sys.argv) > 2:
   outfile = sys.argv[2]
-  f = open(outfile, 'w')
+  f = open(outfile, 'w', encoding='utf-8')
   f.write(xml)
   f.close()
 else: