Improve tooling for repository

[menshikh-iv]

We need to improve the tooling of the repository to increase code-quality and simplify test process

• Add style-checker (pycodestyle, flake8, any)
• Add tox that should allow
  • Run tests with all python version
  • Run style-checkers
  • (optional) Run integration tests

Discussion: @piskvorky @mpenkov wdyt to add automatic code-formatter like https://github.com/google/yapf or https://github.com/ambv/black, I want to drop all discussions about code-style at all (honestly, I want to do it for Gensim, but smart_open are good candidate for test this approach, because it simple and small).

Why I think this is useful:

• auto code-formatting (no need to remember about style always)
• no comments about code-formatting
• always clean code

[piskvorky]

Fine with me of course, good idea. As long as we can enforce, or at least allow, hanging indent (not vertical) and lines longer than 80 chars.

[mpenkov]

I, for one, enjoy flame wars about code style :)

But let's give it a try. If it makes our lives easier, and the code better, then that can't be a bad thing.