Make tools/gn compatible with Python 3

Python 2.7 was end-of-lifed on 2020-01-01. This makes tools/gn
compatible with both Python 2.7 and Python 3.

Issue: flutter/flutter#83043

tools/gn

@@ -66,11 +66,16 @@ def get_out_dir(args):
     return os.path.join(args.out_dir, 'out', '_'.join(target_dir))
 
 def to_command_line(gn_args):
+    """Converts the arguments dictionary to a list of command-line arguments.
+
+    Args:
+      gn_args: GN arguments dictionary generated by to_gn_args().
+    """
     def merge(key, value):
         if type(value) is bool:
             return '%s=%s' % (key, 'true' if value else 'false')
         return '%s="%s"' % (key, value)
-    return [merge(x, y) for x, y in gn_args.iteritems()]
+    return [merge(x, y) for x, y in gn_args.items()]
 
 def cpu_for_target_arch(arch):
   if arch in ['ia32', 'arm', 'armv6', 'armv5te', 'mips',
@@ -327,7 +332,7 @@ def to_gn_args(args):
       gn_args['is_ubsan'] = True
 
     if args.enable_vulkan_validation_layers:
-      if args.target_os is not 'fuchsia':
+      if args.target_os != 'fuchsia':
         print('Vulkan validation layers are currently only supported on Fuchsia targets.')
         sys.exit(1)
       gn_args['enable_vulkan_validation_layers'] = True