Replace patsy with formulaic

[ghislainv]

patsy is a dependency of forestatrisk. patsy is not maintained anymore and has been replaced with formulaic.

patsy should then be replaced by formulaic as a dependency for the forestatrisk package.

More info for formulaic and the migration here:

• https://github.com/pydata/patsy
• https://github.com/matthewwardrop/formulaic
• https://matthewwardrop.github.io/formulaic/migration/