bashtage/linearmodels: Release 4.31

This release adds support for formulaic 0.6.0. This version of formulaic makes a significant change to how variables from formulas are translated into DataFrames for use in models. The existing behavior sorts variables. The new behavior respects the order of the variables as they appear. The new behavior will become the default in linearmodels starting in release 5. To use the new behavior now, add the import from linearmodels.__future__ import ordering to the top of your file