Using a top-level table for `pytest` in `pyproject.toml` offends PEP 518

[abravalheri]

Hi @jaraco, I am under the impression that using a top-level pytest table is not allowed under PEP 518.

pytest-enabler/pytest_enabler/__init__.py

Lines 25 to 27 in 3eaf212

	with open(filename) as strm:
	defn = toml.load(strm)
	return defn["pytest"]["enabler"]

The PEP specifies the tool namespace, so I suppose the best would be reading from tool.pytest.enabler (or tool.pytest-enabler)?

I noticed this issue when I was working on the PEP 621 support for setuptools (and trying to convert setuptools' own setup.cfg file to pyproject.toml).