Update readme for python3 compatibility

Description

It looks like python 3 compatibility was added four months ago: https://github.com/edx/XBlock/commit/4700c7cb5b05fbfbea7d125525b681f9bccc23c3

And XBlock is now being tested by travis in 2.7 and 3.5.

Done

Assignee

Unassigned

Reporter

Open Source Pull Request Bot

Contributor Name

Nik Nyby

Repo

edx/XBlock

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