Update Python version trove classifiers

This commit drops the Python 2.6 trove classifier and adds classifiers
for Python 3.8-3.10, to reflect which versions the automated tests run
on. (The code may still work with Python 2.6 but it's not officially
supported.)

Author: diazona

setup.py

@@ -64,14 +64,16 @@ def run(self):
         'Intended Audience :: Developers',
         'License :: OSI Approved :: MIT License',
         'Programming Language :: Python :: 2',
-        'Programming Language :: Python :: 2.6',
         'Programming Language :: Python :: 2.7',
         'Programming Language :: Python :: 3',
         'Programming Language :: Python :: 3.3',
         'Programming Language :: Python :: 3.4',
         'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
+        'Programming Language :: Python :: 3.8',
+        'Programming Language :: Python :: 3.9',
+        'Programming Language :: Python :: 3.10',
         'Topic :: Software Development :: Testing'
     ]
 )