Scikit-sparse

* Added support for python 3.10 (78)
* Added windows installation instructions (59)

Allow suitesparse include/library paths to be passed in to `pip install` through environment variables.

This release makes no changes to the base code, only the build system.

* Use PEP-517 style build-system so `numpy` and `cython` are no longer needed prior to installation
* Added support for python 3.9