Generate requirements list dynamically instead of using environment markers.
This allows using versions of setuptools older than 20.6.8.

diff -Nurp pdir2-0.3.0.orig/setup.py pdir2-0.3.0/setup.py
--- pdir2-0.3.0.orig/setup.py	2018-02-10 00:03:21.000000000 -0600
+++ pdir2-0.3.0/setup.py	2018-02-28 08:02:50.982899303 -0600
@@ -1,7 +1,15 @@
 import codecs
+import sys
+import platform
 
 from setuptools import setup
 
+requirements = []
+if platform.system == 'Windows':
+    requirements.append('colorama')
+if sys.version_info[:2] < (3, 4):
+    requirements.append('enum34')
+
 readme = codecs.open('docs/README.rst', encoding='utf-8').read()
 history = codecs.open('docs/HISTORY.rst', encoding='utf-8').read()
 
@@ -17,13 +25,9 @@ setup(
         'pdir',
     ],
     setup_requires=[
-        # minimum version to use environment markers
-        'setuptools>=20.6.8',
-    ],
-    install_requires=[
-        'colorama;platform_system=="Windows"',
-        'enum34;python_version<"3.4"',
+        'setuptools',
     ],
+    install_requires=requirements,
     include_package_data=True,
     license='MIT License',
     classifiers=[