Doesn't work on Windows 7

[mac-b-71]

File "\Python\Python36\lib\site-packages\marathon\client.py", line 48, in init
keep_alive = socket_options.TCPKeepAliveAdapter()
File "\Python\Python36\lib\site-packages\requests_toolbelt\adapters\socket_options.py", line 107, in init
(socket.IPPROTO_TCP, socket.TCP_KEEPINTVL, interval),
AttributeError: module 'socket' has no attribute 'TCP_KEEPINTVL'

It seems that attribute TCP_KEEPINTVL was introduced within Windows 10, version 1709.
https://msdn.microsoft.com/pl-pl/library/windows/desktop/ms738596(v=vs.85).aspx

[solarkennedy]

Sorta upstream issue, relates to requests/toolbelt#98