Define code review guidelines for Python codebase

Problem

To simplify review process for authors and reviwers we need to define a clear scopes of what should both parties pay attention to.

We need to define guidelines for maintainers and contributors to Python codebase in GitLab

Requirements

• Define a list of important topics that should everyone pay attention to during code review.
• Describe the linters that we have or should use to automate the review of code styles (for AIGW we use black right now)