avoid private tomlkit method

[dimbleby]

Fixes #4718

I don't know why the code was using the private method _insert_after(), in only this one place. So far as I can see it's unnecessary; but tomlkit remains quite the mystery to me.

Following the repro of #4718 (comment), we now successfully achieve a pyproject.toml structured as follows:

[tool.poetry]
name = "foo"
and = "so on"

[tool.poetry.dependencies]
python = "^3.10"

[tool.poetry.group.dev.dependencies]
pytest = "^7.1.2"

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

[tool.poetry.plugins]

which has put the new entry in a sensible place anyway, so that's good.

Not sure how to write a test for this or that it's even worthwhile. This branch is covered in existing tests so not breaking them is a strong start, the new code is much more obvious than the old code was, I feel that a new testcase would be testing tomlkit more than it would be testing poetry.

[mkniewallner]

Not familiar with tomlkit, but this definitely looks ok to me.

Only thing I can see, which I don't think is related to this change but more to how tomlkit works. Given this file:

[tool.poetry]
name = "aaa"
version = "0.1.0"
description = ""
authors = []

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

[tool.poetry.dependencies]
python = "^3.9"

By running poetry add --group dev pytest, the file will be modified to:

[tool.poetry]
name = "aaa"
version = "0.1.0"
description = ""
authors = []
[tool.poetry.group.dev.dependencies]
pytest = "^7.1.2"


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

[tool.poetry.dependencies]
python = "^3.9"

So there seems to be a mishandling of the carriage return (but again, pretty sure it's not related to the change, just thought it would be worth mentioning).

[dimbleby]

@mkniewallner if you think the missing newline is interesting, please take it to the tomlkit repository

[github-actions]

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