type sparse.py, enable mypy in CI

[frrad]

This is on top of #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

[pchtsp]

hello! thanks. Why the pyproject.toml? and why type hint sparse? also, could you check #755 as this was the current work for the type hinting.

[frrad]

Hello!

I have added pyproject.toml in order to configure mypy. The current configuration is such that mypy . will pass. In particular, the addition I made to your .gihub/... configuration file should successfully run mypy on future PRs.

By introducing this check, we ensure that future files are mypy'd from the start, and give ourselves a nice way to ratchet up the type coverage - you can do smaller PRs which remove one or two files from pyproject.toml and add types to them. I chose sparse.py because it was an easy starting point.

I did notice #755

1. since sparse.py is so isolated you should just be able to "take theirs" when you merge this branch into it and have no conflicts with type annotations in other files.
2. I think that PR is a compelling piece of evidence in favor of the incremental approach - instead of having a years-long back and forth over one monolithic change you can just take small improvements as they come with less back and forth.

[pchtsp]

Thanks, I've left a couple more comments and we should merge afterwards.

[pchtsp]

please leave the versions open. That is:

black
mypy

[pchtsp]

Can you just add a directory instead of each file? something like seems to work according to https://stackoverflow.com/a/78480102/6508131:

exclude = [
"examples/",
"doc",
"pulp/apis",
"pulp/solverdir",
]

[frrad]

Thanks for merging!

FYI the disadvantage of blacklisting folders is that new files added in these folders will be blacklisted be default.

Another option if you don't like the long list is to just add ignore comments to the tops of all the existing files. I'm happy to implement either of these as a separate PR if you're interested.