Run pyupgrade for Python 3.6+
Hi ! Thanks for the change :)
Could it be possible to only run it for the code in `src/` ? We try to not change the code in the `datasets/` directory too often since it refreshes the users cache when they upgrade `datasets`. > Hi ! Thanks for the change :)
> Could it be possible to only run it for the code in `src/` ? We try to not change the code in the `datasets/` directory too often since it refreshes the users cache when they upgrade `datasets`.

I reverted the changes in `datasets/` instead of changing only `src/`. Does it sound good? I just resolved some conflicts with the master branch. If the CI is green we can merge :)
Run the command:

```bash
pyupgrade $(find . -name "*.py" -type f) --py36-plus
```

Which mainly avoids unnecessary lists creations and also removes unnecessary code for Python 3.6+.

It was originally part of #3489.

Tip for reviewing faster: use the CLI (`git diff`) and scroll.