fix win-inet-pton conditional requirement

requirements.txt

@@ -8,4 +8,4 @@ gevent-eventemitter~=2.1
 PyYAML==5.1
 cachetools>=3.0.0
 enum34==1.1.2; python_version < '3.4'
-win-inet-pton; python_version == '2.7' and platform_system == 'Windows'
+win-inet-pton; python_version == '2.7' and sys_platform == 'win32'

setup.py

@@ -19,7 +19,7 @@ install_requires = [
     'requests>=2.9.1',
     'vdf>=3.3',
     'cachetools>=3.0.0',
-    "win-inet-pton; python_version == '2.7' and platform_system == 'Windows'",
+    "win-inet-pton; python_version == '2.7' and sys_platform == 'win32'",
     "enum34==1.1.2; python_version < '3.4'",
 ]
 

steam/utils/__init__.py

@@ -2,14 +2,14 @@
 """
 import six
 from six.moves import xrange as _range
+import sys
 
-if six.PY2:
+if six.PY2 and sys.platform == 'win32':
     import win_inet_pton
 
 import weakref
 import struct
 import socket
-import sys
 
 
 def ip4_from_int(ip):