build-swift: Explicitly use python3

This changes build-swift script shebang to use Python 3 explicitly. The
code is already compatible and functional with python3, so this removes
any implied backward compatibility, and removes any ambiguity based on
the user's current environment, especially since some systems, like
macOS, still link 'python' to Python 2.

This also removes the now unnecessary 'from __future__' imports, and the
'six' module, which is only necessary for python2 compatibility.

Author: jorng

utils/build-script

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # This source file is part of the Swift.org open source project
 #
@@ -8,14 +8,10 @@
 # See https://swift.org/LICENSE.txt for license information
 # See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 
-
 """
 The ultimate tool for building Swift.
 """
 
-
-from __future__ import absolute_import, print_function, unicode_literals
-
 import json
 import os
 import platform
@@ -32,8 +28,6 @@ from build_swift.build_swift.constants import SWIFT_BUILD_ROOT
 from build_swift.build_swift.constants import SWIFT_REPO_NAME
 from build_swift.build_swift.constants import SWIFT_SOURCE_ROOT
 
-import six
-
 from swift_build_support.swift_build_support import build_script_invocation
 from swift_build_support.swift_build_support import shell
 from swift_build_support.swift_build_support import targets
@@ -119,7 +113,7 @@ class JSONDumper(json.JSONEncoder):
     def default(self, o):
         if hasattr(o, '__dict__'):
             return vars(o)
-        return six.text_type(o)
+        return str(o)
 
 
 def print_xcodebuild_versions(file=sys.stdout):
@@ -506,7 +500,7 @@ def main_preset():
     try:
         preset_parser.read_files(args.preset_file_names)
     except presets.PresetError as e:
-        fatal_error(six.text_type(e))
+        fatal_error(str(e))
 
     if args.show_presets:
         for name in sorted(preset_parser.preset_names,
@@ -527,7 +521,7 @@ def main_preset():
             args.preset,
             vars=args.preset_substitutions)
     except presets.PresetError as e:
-        fatal_error(six.text_type(e))
+        fatal_error(str(e))
 
     preset_args = migration.migrate_swift_sdks(preset.args)