Consider relaxing upper pin on openff-toolkit

[mikemhenry]

Right now we can't install the latest openff-toolkit (0.15.1) with openfe since we have a pin: openff-toolkit >=0.13,<0.15

Looking at the changelog https://docs.openforcefield.org/projects/toolkit/en/stable/releasehistory.html some oof the bug fixes in the 0.15.x line are useful, no relevant API breaks, and enables the use of modern QCFractal.

I propose we change the pin to >=0.13,<0.16 so we can guard against future major API breaks, but allow users to install the latest and greatest openff-toolkit

Let me know your thoughts @richardjgowers @IAlibay @hannahbaumann @dwhswenson @RiesBen