Include py.typed in exported wheels and sdists - #98

I didn't check what the version was in my original venv, but it was working there originally, and I checked it again now. I don't know what changed since then, but in a new venv, this problem does occur and this fixes it.

@ksunden did you check matplotlib against this version? IOW, should I tag an rc and update matplotlib/matplotlib#26943 to use it, or can I just do a point release directly?

Yes I have tested against matplotlib.

In a fresh virtualenv, the only errors I got were due to pytest not being installed from requirements/testing/mypy.txt, so that should be added.

I'd be fine with just going straight to a point release.

And then adding pytest in matplotlib/matplotlib#26943 to the requirements file (along with removing the skipping from pyproject.toml). (This also escapes one error due to pytest.skip not being picked up as terminal and thus not type-narrowing __doc__)

I did at one point get some errors from pytest itself, but could never get that again after clearing caches, and they were nothing to do with mpl/cycler (though was a missing import of argcomplete).