Handle pinned version of isort
The CONTRIBUTING file pins to a specific version of isort, so we might as well install that in `dev` . This makes it easier for contributors so they don't have to manually install the specific commit.
I didn't know that worked, thanks @BramVanroy. @aaugustin what do you think?<|||||>I'm not familiar with the syntax, but if it works, go for it. I really hope we have a release of sort and we can remove this soon.<|||||>Works on my machine so I'll merge :)

Thanks @BramVanroy, this will simplify @LysandreJik and @thomwolf's lives a lot!