[FIX] flake8

Using the png bage for coverage allows to specify which branch to show.
```
[![Coverage Status](https://coveralls.io/repos/OCA/web/badge.png?branch=6.1)](https://coveralls.io/r/OCA/web?branch=6.1)
```
[![Coverage Status](https://coveralls.io/repos/OCA/web/badge.png?branch=6.1)](https://coveralls.io/r/OCA/web?branch=6.1)