Update m2r to 0.3.1

[pyup-bot]

This PR updates m2r from 0.2.1 to 0.3.1.

Changelog

0.3.0

* Drop support for Python 2.7, 3.4, 3.5, and 3.6
* Add compatibility with docutils 0.19
* Sync up assertion with changes in argparse
* Limit mistune dependency version range

Links

• PyPI: https://pypi.org/project/m2r
• Changelog: https://pyup.io/changelogs/m2r/
• Repo: https://github.com/miyakogi/m2r