Remove setup.cfg in favor of pyproject.toml

[j-wags]

In #1822 we tried to move up to pytest 8, which involved moving the slow test marker out of _tests/conftest.py and into the top level pyproject.toml. It turned out that this caused something to go wrong with our nightly tests which tried to use said marker and led to a failure during test collection, as documented in #1997. In #2017 this was partially reverted by putting the faulty marker in the top level setup.cfg instead of pyproject.toml. We should take another go at consolidating the marker definition from setup.cfg into pyproject.toml.