Specify conditional dependencies such that they're recognized by bdist_wheel.

On Windows, use kerberos-sspi because it's much easier to get installed.
On other platforms, use pykerberos instead of kerberos for Python 3 support.

requirements.txt

@@ -1 +1,3 @@
 requests>=1.1.0
+kerberos-sspi >= 0.2; sys.platform == 'win32'
+pykerberos >= 1.1.1, < 2.0.0; sys.platform != 'win32'

setup.py

@@ -1,18 +1,9 @@
 #!/usr/bin/env python
 # coding: utf-8
 import os
-import sys
 import re
 from setuptools import setup
 
-with open('requirements.txt') as requirements:
-    requires = [line.strip() for line in requirements if line.strip()]
-
-if sys.platform == 'win32':
-    requires.append('kerberos-sspi')
-else:
-    requires.append('kerberos==1.1.1')
-
 path = os.path.dirname(__file__)
 desc_fd = os.path.join(path, 'README.rst')
 hist_fd = os.path.join(path, 'HISTORY.rst')
@@ -56,7 +47,13 @@ setup(
     package_data={'': ['LICENSE', 'AUTHORS']},
     include_package_data=True,
     version=get_version(),
-    install_requires=requires,
+    install_requires=[
+        'requests>=1.1.0',
+    ],
+    extras_require={
+        ':sys_platform=="win32"': ['kerberos-sspi>=0.2'],
+        ':sys_platform!="win32"': ['pykerberos>=1.1.1,<2.0.0'],
+    },
     test_suite='test_requests_kerberos',
     tests_require=['mock'],
 )