FIX: set intermediate TOML tables to be super table

[redeboer]

The create_sub_table() function would sometimes result in a structure like the following. This is now fixed.

import tomlkit
from compwa_policy.utilities.pyproject import load_pyproject_toml
from compwa_policy.utilities.pyproject.setters import create_sub_table

pyproject = load_pyproject_toml("", modifiable=True)
pixi = create_sub_table(pyproject, "tool.pixi")
pixi["channels"] = ["conda-forge"]
toml_src = tomlkit.dumps(pyproject)

[tool]  # should be a super table
[tool.pixi]
channels = ["conda-forge"]