PEP-518 introduced changes not in Python itself but rather in its ecosystem. The idea is pretty simple: let's store configs for all tools in pyproject.toml file, in tool.TOOL_NAME section. For example, for mypy:

[tool.mypy]
files = ["my_project"]
python_version = 3.8

At this moment, almost all popular tools support pyproject.toml as the configuration file, in one way or another: mypy, pytest, coverage, isort, bandit, tox, etc. The only exception from the tooling I know is flake8.

Before pyproject.toml, many tools used to use setup.cfg for the same purpose, but this format (INI) has a few disadvantages compared to TOML: it's not well-standardized, and the only supported type of values is string.