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- Add support for Python 3.6, 3.7, and 3.8.
- Drop support for Python 3.3.
- Fix travis configuration.
- Add ``samesite`` option to AuthTktCookiePlugin constructor.
If this is passed, it should be a string, and it will be used
to compose the Set-Cookie header's "SameSite" value, e.g.
if you pass ``samesite="Strict"`` into the constructor,
the cookie value for the auth tkt cooke will contain
``SameSite=Strict``.