CLN: move pytest config to pyproject.toml
According to https://docs.pytest.org/en/stable/customize.html, using `setup.cfg`as pytest config file is not recommended. So moved everything to `pyproject.toml`

I'm reticent to do this.  The everything-in-one-file nature of setup.cfg is really nice improvement over the older proliferation of config files > I'm reticent to do this. The everything-in-one-file nature of setup.cfg is really nice improvement over the older proliferation of config files

Isn't pyproject.toml the "new" everything-in-one file? I would love to be able to use `poetry update` in the source tree to get all dependencies installed/updated (would need to move all the depdencies to pyproject.toml). can we move *everything* to pyproject.toml? (or is this not supported for some things yet)? > can we move _everything_ to pyproject.toml? (or is this not supported for some things yet)?

We can also move `isort` to pyproject.toml. But other tools don't support it yet.  And seems like mypy is going to support pyproject.toml soon https://github.com/python/mypy/pull/10219. Black doesn't support setup.cfg, flake8 doesn't support pyproject.toml, and the xkcd comic about standards is as relevant as ever 😆 

Anyway if the pytest docs advise against setup.cfg for pytest, then this PR seems like a good idea, thanks for doing it LOL... Let me put it here.

![](https://imgs.xkcd.com/comics/standards.png)
 this looks like a net improvement to me, sure we can't move everything yet, but newer versions you can (e.g. isort)

cc @jorisvandenbossche @jbrockmendel  as long as we have someone who is competent with toml to maintain this file, no objection here hmm somehow a conflict, can you merge master Thanks @fangchenli ! @fangchenli running the tests locally, test-data.xml is generated and is an untracked file. probably need to add to git ignore to clean up.