Add a tridiagonal matrix to the benchmarks

[Ericgig]

We have benchmarks for random sparse and dense matrices, but both are bad scenario for the Dia format.
I added a tridiagonal matrix case to benchmark the Dia operations in situation where they are useful.

[hodgestar]

There is a minor black formatting issue (easily fixed) and it looks like mambaforge needs replacing with miniforge in this repo too, but the actual change looks good.