ordereddict on Python 2.6, singledispatch on all Python ver- sions prior to 3.4 (This was an optional dependency in prior versions of Tornado, and is now mandatory), and backports_abc>=0.4 on all versions prior HTTPServer) now accept either a dictionary of options or an ssl.SSLContext object. • New optional dependency on concurrent.futures to provide better support for working with threads. concurrent.futures is

ordereddict on Python 2.6, singledispatch on all Python versions prior to 3.4 (This was an optional dependency in prior versions of Tornado, and is now mandatory), and backports_abc>=0.4 on all versions prior ssl.SSLContext [https://docs.python.org/3/library/ssl.html#ssl.SSLContext] object. New optional dependency on concurrent.futures [https://docs.python.org/3/library/concurrent.futures.html#module-concurrent