Adjust astropy dependency depending on Python version (issue #12)

setup.py

 from __future__ import print_function
 from setuptools import setup, Command
 import os
+import sys
 import lsmtool._version
 
 
@@ -24,6 +25,11 @@ class PyTest(Command):
         errno = subprocess.call([sys.executable, 'runtests.py'])
         raise SystemExit(errno)
 
+# Handle Python 3-only dependencies
+if sys.version_info < (3, 0):
+    reqlist = ['numpy','astropy >= 0.4, <3.0']
+else:
+    reqlist = ['numpy','astropy >= 0.4']
 
 setup(
     name='lsmtool',
@@ -43,7 +49,7 @@ setup(
         'Topic :: Scientific/Engineering :: Astronomy',
         'Topic :: Software Development :: Libraries :: Python Modules',
         ],
-    install_requires=['numpy','astropy >= 0.4'],
+    install_requires=reqlist,
     scripts=['bin/lsmtool'],
     packages=['lsmtool','lsmtool.operations'],
     setup_requires=['pytest-runner'],