Switch from Patsy to Formulaic #173
Description
Patsy breaks once in a while, while Formulaic is a competitor that is more actively-developed, and supports more features, seemingly including differentiation which would help simplify some of the code in formulation.py that extends Patsy with some SymPy functionality.
This is likely low priority at the moment, since Patsy only breaks rarely, and a lack of formula flexibility is not a common complaint. But it could be a good first contribution for someone interested in improving PyBLP and getting familiar with its codebase.