Port source to Python 3 (pantsbuild#6197)

Part of pantsbuild#6062

src/python/pants/source/BUILD

@@ -4,6 +4,7 @@
 python_library(
   sources = globs('*.py', exclude=[globs('payload_fields.py')]),
   dependencies=[
+    '3rdparty/python:future',
     '3rdparty/python:six',
     '3rdparty/python/twitter/commons:twitter.common.dirutil',
     'src/python/pants/base:build_environment',

src/python/pants/source/source_root.py

@@ -5,10 +5,9 @@
 from __future__ import absolute_import, division, print_function, unicode_literals
 
 import os
+from builtins import object, range
 from collections import namedtuple
 
-from six.moves import range
-
 from pants.base.project_tree_factory import get_project_tree
 from pants.subsystem.subsystem import Subsystem
 from pants.util.memo import memoized_method, memoized_property

src/python/pants/source/wrapped_globs.py

@@ -269,7 +269,7 @@ class Files(FilesetRelPathWrapper):
 
   @staticmethod
   def _literal_files(*args, **kwargs):
-    if kwargs.keys() != ['root']:
+    if list(kwargs.keys()) != ['root']:
       raise ValueError('Literal file globs do not support kwargs other than `root`: {}'.format(kwargs))
     return args
 

tests/python/pants_test/source/test_wrapped_globs.py

@@ -254,7 +254,7 @@ def test_iter_relative_paths(self):
     efws = self.sources_for(['a', 'b', 'c'], 'test_root')
     self.assertEquals(
       efws.files_hash,
-      str('cb11a7f0b5a1e22b93c36783608ba531ea831c2f68a5c9f9498417b211bcfea4'),
+      'cb11a7f0b5a1e22b93c36783608ba531ea831c2f68a5c9f9498417b211bcfea4',
     )
     self.assertEquals(
       list(efws.paths_from_buildroot_iter()),