Enforce Twisted<15.5.0 for python 2.6
Enforce Twisted<15.5.0 for python 2.6

15.5.0 is the last release supporting python 2.6 [0]... but actually
doesn't support it [1].
Let's make it <15.5.0
[0] https://twistedmatrix.com/trac/browser/tags/releases/twisted-15.5.0/NEWS?format=raw
[1] https://twistedmatrix.com/trac/browser/tags/releases/twisted-15.5.0/twisted/__init__.py#L16

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