win => win32

To determine the window, check the win32. With the verification that is now, there will be problems in the Mac OS:
$ python
Python 3.7.4 (default, Jul  9 2019, 18:15:00)
[Clang 10.0.0 (clang-1000.11.45.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> sys.platform
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'sys' is not defined
>>> import sys
>>> sys.platform
'darwin'
>>>
$ sw_vers
ProductName:  Mac OS X
ProductVersion:  10.14
BuildVersion:  18A391

https://docs.python.org/3/library/sys.html#sys.platform

Author: Woklex

setup.py

@@ -32,7 +32,7 @@
         'six',
         'oauthlib',
         'requests_oauthlib'
-    ] + (['kerberos-sspi'] if "win" in sys.platform else ['kerberos']),
+    ] + (['kerberos-sspi'] if "win32" in sys.platform else ['kerberos']),
     platforms='Platform Independent',
 
     classifiers=[