Bump Python version classifiers

Commit 429ac946 relies on order-preserving dicts, i.e. Python 3.7+

setup.py

@@ -9,8 +9,8 @@ setuptools.setup(
 	classifiers = [
 		'Development Status :: 3 - Alpha',
 		'License :: OSI Approved :: GNU General Public License v3 or later (GPLv3+)',
-		'Programming Language :: Python :: 3.6',
 		'Programming Language :: Python :: 3.7',
+		'Programming Language :: Python :: 3.8',
 	],
 	packages = ['qwarc'],
 	setup_requires = ['setuptools_scm'],