update a docstring and add a str import for py3

src/python/pants/backend/native/subsystems/native_build_step_settings_base.py

@@ -10,6 +10,12 @@
 
 
 class NativeBuildStepSettingsBase(Subsystem, MirroredTargetOptionMixin):
+  """Holds options which are understood by all parts of the compile and link pipeline.
+
+  NB: This is separate from NativeBuildSettings, which is used to store options controlling pants
+  behavior. Rather, this subsystem stores options which are typically interpreted into command-line
+  arguments or environment variables for a compiler or linker invocation.
+  """
 
   mirrored_option_to_kwarg_map = {
     'fatal_warnings': 'fatal_warnings',

src/python/pants/option/global_options.py

@@ -7,6 +7,7 @@
 import multiprocessing
 import os
 import sys
+from builtins import str
 
 from pants.base.build_environment import (get_buildroot, get_default_pants_config_file,
                                           get_pants_cachedir, get_pants_configdir, pants_version)