Remove legacy poetry sections from pyproject.toml

[sonotley]

Use the newer PEP621 standard. Possibly move away from templating this in favour of allowing poetry to generate it.

[sonotley]

The latest poetry still creates [tool.poetry] and [tool.poetry.dependencies] even though I think there are PEP-621 variants for these, so need to check current status of PEP-621 support in poetry. A quick comparison shows that Hatch uses PEP-621 sections so perhaps a reason to consider a Hatch variant.