Update setup.py

sorry, Debian maintainers imaginary or actual

setup.py

@@ -10,7 +10,7 @@ setup(name='Mastodon.py',
       description='Python wrapper for the Mastodon API',
       packages=['mastodon'],
       setup_requires=['pytest-runner'],
-      install_requires=['requests', 'python-dateutil', 'six', 'pytz', 'decorator'],
+      install_requires=['requests', 'python-dateutil', 'six', 'pytz', 'decorator>=4.0.0'],
       tests_require=test_deps,
       extras_require=extras,
       url='https://github.com/halcy/Mastodon.py',