blackdoc: apply black to code in documentation

blackdoc extracts code from documentation, applies black to it and, in reformatting mode, writes the formatted code back to the file.

The currently supported formats are:

• doctest

• ipython

• rst

• markdown (python blocks only for now, no pycon blocks)

Documentation

User Guide

• Installation

• Usage

• Options

Help & Reference

• Changelog

• Contributing