Add Python classifiers to setup.py

Add Python classifiers to setup.py to include Python, Python 2 and
Python 3. For more information about classifiers see:
https://www.python.org/dev/peps/pep-0301/#distutils-trove-classification

Author: roaldnefs

setup.py

@@ -12,4 +12,9 @@
     keywords         = ['messagebird', 'sms'],
     install_requires = ['requests>=2.4.1'],
     license          = 'BSD-2-Clause',
+    classifiers      = [
+        'Programming Language :: Python',
+        'Programming Language :: Python :: 2',
+        'Programming Language :: Python :: 3',
+    ],
 )