Remove check for node.docstring

[berkerpeksag]

node.docstring has just removed from 3.7.0.b5 due to backwards compatibility reasons. See python/cpython#7121 for details.

By looking at the pull request, reverting 524d525 might be enough.

@radomirbosak would you like to submit a pull request?

[berkerpeksag]

We should, of course, still keep the tests and changes in .travis.yml.

[Kodiologist]

Beware that only old alphas of Python 3.7 are currently available on Travis.

[berkerpeksag]

Thanks for the heads up. The whole docstring situation is a complete mess...