Package license info is non-conformant with modern standards

[GergelyKalmar]

Describe the bug:
The current package license info is considered deprecated (see https://packaging.python.org/en/latest/specifications/core-metadata/#license, https://packaging.python.org/en/latest/specifications/core-metadata/#license-expression and https://packaging.python.org/en/latest/specifications/core-metadata/#license-file-multiple-use).

This causes issues when using license checking tools which typically rely on the now-standard PEP639-style SPDX expressions.

[BenRKarl]

@GergelyKalmar thank you, we'll take a look and get it fixed.