Remove dependency on isort

[fpgmaas]

Suggestion by @mkniewallner:

Currently, isort is a dependency since it is used to get the standard library's for various python versions. We can simply generate and store these lists directly in deptry ourselves, to reduce deptry's dependencies and thereby the risk of conflicting dependencies for end users. isort generates them with this script. We could use that script to create the files within deptry itself.

Additional context

If we do not implement this solution, we might limit the dependency on isort to <3.10, since 3.10 has a built-in function to fetch the standard library.