Fix license in pyproject.toml

[mjdemilliano]

This change makes pyproject.toml compliant to PEP-621 w.r.t. license specification.

Update: changed to correct reference.

[roberthdevries]

The current specification for python package information that specifies what goes into a pyproject.toml file can be found in pyproject.toml specification
PEP 639 has been superseded by the above spec. Assuming that the above-mentioned PEP 612 is a mistake as it discusses Parameter Specification Variables related to typing.

The proposed changes use a construct marked as legacy.

[mjdemilliano]

You're right. The number should have been PEP-621 and has indeed been superseded. Discussion to be held about which PEP this repository still has to support.

[sebastian-carpenter]

@mjdemilliano do you need support for the legacy version? The current code seems to reflect the standard and I'm not sure there's an easy way to support both.

[mjdemilliano]

I currently still need it but doesn't have to be upstream, so if you are ok with not supporting the legacy version then let's not change it back.