Should we have something like this in matplotlib for the old freetype interface?

We could: it would make it easier to do when installing from pip, for example. In the case of matplotlib though, it's somewhat less necessary: the only people who need to do this are developers, and they would be on a git checkout anyway. But certainly I'm not opposed, and it's easy enough to add.

I was thinking about this in connection with matplotlib/matplotlib#4678 because I changed that to pip install -e . on matplotlib but actually we don't really need it since modifying setup.cfg works when you are installing with pip from a source checkout.

It would perhaps make sense for tests in basemap, cartopy or something else that depends on matplotlib and uses image comparison tests.