Add Trove MacOS classifiers

Altough this library is OS-independent, it is specifically relevant to
Classic Mac OS and OS X, and provides some extra functionality on OS X
(reading actual resource forks, instead of resource fork data stored in
data forks/regular files).

setup.cfg

@@ -10,6 +10,8 @@ classifiers =
 	Topic :: System
 	Topic :: Utilities
 	License :: OSI Approved :: MIT License
+	Operating System :: MacOS :: MacOS 9
+	Operating System :: MacOS :: MacOS X
 	Operating System :: OS Independent
 	Programming Language :: Python
 	Programming Language :: Python :: 3