Add python_requires so package installers know this requires 3.7 or newer (#494)

Also remove some stale classifiers

Author: cjw296

setup.py

@@ -26,15 +26,13 @@
     },
     author='Alec Thomas',
     author_email='alec@swapoff.org',
+    python_requires=">=3.7",
     classifiers=[
         'Development Status :: 5 - Production/Stable',
         'Intended Audience :: Developers',
         'License :: OSI Approved :: BSD License',
         'Operating System :: OS Independent',
-        'Programming Language :: Python :: 2',
-        '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',