setup: import setup from setuptools if FORCE_SETUPTOOLS is set

This should allow easier experimentation with using setuptools in mercurial's
build automation, without breaking anything that currently depends on distutils
behavior

setup.py

@@ -70,7 +70,11 b' import re'
 import shutil
 import tempfile
 from distutils import log
-from distutils.core import setup, Command, Extension
+if 'FORCE_SETUPTOOLS' in os.environ:
+    from setuptools import setup
+else:
+    from distutils.core import setup
+from distutils.core import Command, Extension
 from distutils.dist import Distribution
 from distutils.command.build import build
 from distutils.command.build_ext import build_ext