Implement PEP 639

[befeleme]

Issue Kind

Change in current behaviour

Description

See: https://peps.python.org/pep-0639

Additional context:

• The full implementation, including the Core Metadata bump to 2.4, will require also the support on the side of PyPI first:
  Implement PEP 639 pypi/warehouse#16620

• There's a plan to add validation data and parser to packaging:
  Add SPDX license data pypa/packaging#799 + PEP 639: Implement License-Expression and License-File pypa/packaging#828

Impact

poetry now uses the license field from the custom [tool.poetry] table - this will enable unification under the specified main [project] table of pyproject.toml and map to a new core metadata field.
This also brings in a standardized way of specifying the license files in the package metadata, which will help the downstream consumers locate them unambiguously.

Workarounds

poetry now includes both the license expression (as SPDX) and the license files in the distribution. This will just make the declarations and metadata unified across many packaging tools.