Add Linux classifier

The WARC writing is only tested and known safe on Linux and relies on its behaviour of flock(2) and files opened in append mode (writes always append regardless of the stream position).

setup.py

@@ -11,6 +11,7 @@ setuptools.setup(
 		'License :: OSI Approved :: GNU General Public License v3 or later (GPLv3+)',
 		'Programming Language :: Python :: 3.7',
 		'Programming Language :: Python :: 3.8',
+		'Operating System :: POSIX :: Linux',
 	],
 	packages = ['qwarc'],
 	setup_requires = ['setuptools_scm'],