Update README for WAMP example to use crossbar with python3

crossbar python2 support is broken in v18.7.2.
I've submitted with fixes for that.
https://github.com/crossbario/crossbar/pull/1376

But they don't test crossbar with python2 in CI, so it's going to be
broken again in the future.
And there is no much reason to use crossbar with python2.

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