Update pyproject.toml to be compatible with poetry 2

[alightwing]

The relevant poetry 2 breaking changes are all to do with how the pyproject.toml is formatted:

• It uses the [project] block properly now as per PEP 621, resolving a long-standing complaint we've had (see deleted comment).
  • This means we don't need the separate [tool.poetry] block any more.
  • However, it's very specific about what it allows to have in its license fields, hence the splitting of the existing license field into license and license-files.
• It doesn't have a specific concept of dev dependencies any more, instead making those a generic group of sub-requirements.

Additionally, when running the unittests after these changes, pyfakefs went completely bonkers with some kind of infinite recursion bug, so a version bump was necessary in order to fix that.

I've verified this works with both Python 3.8 and 3.13, so we don't need to bump minimum Python version (yet).

SMP-3234

[alightwing]

Okay, looks like we are going to have to bump minimum Python version as 3.8 can only pip3 install poetry version 1.8.5 - has no access to Poetry 2.

[kisoso]

Thanks.