setup.py: Support older setuptools (<34.x)

The `python_version` conditional was added fairly recently (<2 years).
We've run into a few cases where it would be very convenient for luigi
to support older setuptools (e.g. managed environments where it may be
difficult or not possible to upgrade).

setup.py

@@ -13,6 +13,7 @@
 # the License.
 
 import os
+import sys
 
 from setuptools import setup
 
@@ -41,9 +42,13 @@ def get_static_files(path):
     # https://pagure.io/python-daemon/issue/18
     'python-daemon<2.2.0',
     'python-dateutil==2.7.5',
-    'enum34>1.1.0;python_version<"3.4"',
 ]
 
+# Note: To support older versions of setuptools, we're explicitly not
+#   using conditional syntax (i.e. 'enum34>1.1.0;python_version<"3.4"').
+if sys.version_info[:2] < (3, 4):
+    install_requires.append('enum34>1.1.0')
+
 if os.environ.get('READTHEDOCS', None) == 'True':
     # So that we can build documentation for luigi.db_task_history and luigi.contrib.sqla
     install_requires.append('sqlalchemy')