Fix import sorting in create_baseline_stubs script

[JukkaL]

The isort --recursive option doesn't recognize .pyi files, so find all
.pyi files explicitly.

Run isort before black since changes made by isort don't follow stub
conventions.

[srittau]

I always run isort on the stub dir itself, without the --recursive option, which isn't required since isort 5. I would try that first.

[JukkaL]

It turns that the problem was that for some reason isort runs a pre-5 version for me. I still don't quite understand what is going on. I'm going to update this to use python3 -m isort, since it works for me.