type `sparse.py`

[frrad]

This is on top of https://github.com/coin-or/pulp/pull/772 - the diff will be a little cleaner once that is merged.

This PR:

1. adds type hints for sparse.py. I have made no functional changes, only types.
2. adds a pyproject.toml which blacklists all files which don't currently pass mypy (this is most of them)
3. changes .github/workflows/pythonpackage.yml to run mypy
4. removes the (I believe) unused .travis.yml config