* fix confused flake

We run `black  --target-version py35 ...` but flake8 doesn't know that, so currently with py38 flake8 fails suggesting that black should have reformatted 63 files. Indeed if I run:

```
black --line-length 119 --target-version py38 examples templates tests src utils
```
it indeed reformats 63 files.

The only solution I found is to create a black config file as explained at https://github.com/psf/black#configuration-format, which is what this PR adds.

Now flake8 knows that py35 is the standard and no longer gets confused regardless of the user's python version.

* adjust the other files that will now rely on black's config file