Add LP solver backends for HiGHS via scipy.optimize.linprog

[mkoeppe]

scipy 1.6 added a vendored copy of the HiGHS LP solvers. ​https://github.com/scipy/scipy/tree/master/scipy/optimize/_highs/src

​https://docs.scipy.org/doc/scipy/reference/optimize.linprog-highs.html

We should start using them.

scipy 1.7 makes dual values available - ​https://docs.scipy.org/doc/scipy/reference/release.1.7.0.html#scipy-optimize-improvements

​https://www.maths.ed.ac.uk/hall/HiGHS/ also claims to have MIP, but this does not seem to be exposed in scipy.optimize. Upstream issue ​https://github.com/scipy/scipy/issues/14455 tracks the task of adding a MIP interface.

highs also has a rudimentary python interface in its own source tree - ​https://github.com/ERGO-Code/HiGHS/blob/master/src/interfaces/highs_python_api.py