Setuptools has been transitioning away from old path-based pth files for

editable builds to newer import hooks, but these aren't compatible with

tools like mypy, pyright, or IDEs. Because of this, there's expected

to be long-term support for activating a "compat" editable mode, which

we now opt into using our build backend. This ensures typing for all

editable builds.

Tested installing with and without the compat editable mode. In both

cases, we get an __editable__.Djblets-<version>.pth file in the

site-packages directory. Previously, that'd contain an import,

but now it lists a path.