setup: Declare supported Python versions (2.4-2.7, 3.6+)

setup.py

@@ -48,6 +48,7 @@ setup(
     license = 'New BSD',
     url = 'https://github.com/mitogen-hq/mitogen/',
     packages = find_packages(exclude=['tests', 'examples']),
+    python_requires='>=2.4, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*, !=3.5.*, <4',
     zip_safe = False,
     classifiers = [
         'Environment :: Console',
@@ -55,11 +56,16 @@ setup(
         'License :: OSI Approved :: BSD License',
         'Operating System :: POSIX',
         'Programming Language :: Python',
+        'Programming Language :: Python :: 2',
         'Programming Language :: Python :: 2.4',
         'Programming Language :: Python :: 2.5',
         'Programming Language :: Python :: 2.6',
         'Programming Language :: Python :: 2.7',
+        'Programming Language :: Python :: 3',
         'Programming Language :: Python :: 3.6',
+        'Programming Language :: Python :: 3.7',
+        'Programming Language :: Python :: 3.8',
+        'Programming Language :: Python :: 3.9',
         'Programming Language :: Python :: Implementation :: CPython',
         'Topic :: System :: Distributed Computing',
         'Topic :: System :: Systems Administration',