heroke deployment failed on "assertionError: steem requires Python 3.6 or newer"

[p-alik]

assert exception appears during deployment. It looks like steem-python requires newer python https://github.com/steemit/steem-python/issues/79

Update runtime.txt fixed the issue for me.

Deployment log snippet:

-----> Python app detected

 !     The latest version of Python 3 is python-3.6.4 (you are using python-3.6.2, which is unsupported).

 !     We recommend upgrading by specifying the latest version (python-3.6.4).

       Learn More: https://devcenter.heroku.com/articles/python-runtimes

-----> Installing python-3.6.2

-----> Installing pip

-----> Installing requirements with pip

[charlesjlee]

thanks for the PR!