Fix conditional requirement on Windows

Author: peterbrittain

CHANGES.rst

@@ -1,6 +1,10 @@
 CHANGE HISTORY
 ==============
 
+1.7.2
+-----
+- Fixed Windows dependencies
+
 1.7.1
 -----
 - Fixed Linux distribution

setup.py

@@ -61,7 +61,10 @@
         'pyfiglet >= 0.7.2',
         'Pillow >= 2.7.0',
         'future',
-    ] + (['pypiwin32'] if sys.platform == 'win32' else []),
+    ],
+    extras_require={
+        ':sys_platform == "win32"': ['pypiwin32'],
+    },
     setup_requires=['setuptools_scm'],
     tests_require=[
         'mock',