invalid toml for differing extras

pyproject.toml

[tool.poetry]
name = "extras_gist"
version = "0.1.0"
description = ""
authors = ["delphyne <[email protected]>"]

[tool.poetry.dependencies]
python = "^3.6"
pytest = "*"
pytest = {version = "*", extras = "testing", optional = true}

[tool.poetry.extras]
foo = ["pytest"]

[tool.poetry.dev-dependencies]


[build-system]
requires = ["poetry>=0.12"]
build-backend = "poetry.masonry.api"