Update dependency handling for Python 3.

 - In setup.py don't depend on paste if Python 3.
 - All development dependencies are now compatible with Python 2 and Python 3, remove dev-requirements3.txt.

dev-requirements3.txt

-# Optional requirements used by test cases
-## pycurl (Latest released version does not work with Python 3)
-git+git://github.com/pycurl-devs/pycurl
-
-# For testing
-nose
-webtest
-coverage
-
-# For dev
-sphinx
-
-pyflakes

setup.py

 import re
 import ast
+import sys
+
 try:
     from distutils.util import get_platform
     is_windows = get_platform().startswith("win")
@@ -7,7 +9,6 @@ except ImportError:
     # Don't break install if distuils is incompatible in some way
     # probably overly defensive.
     is_windows = False
-
 try:
     from setuptools import setup
 except ImportError:
@@ -21,11 +22,13 @@ requirements = [
     'six',
     'webob',
     'psutil',
-    'paste',
-    'PasteScript',
-    "pyyaml",
+    'pyyaml',
 ]
 
+if sys.version_info[0] == 2:
+    requirements.append('PasteScript')
+    requirements.append('paste')
+
 test_requirements = [
     # TODO: put package test requirements here
 ]