Add SPDX license data

[ofek]

In response to this comment by a maintainer expressing approval for license parsing functionality, this PR introduces an invoke task to update an enumeration encoded as a Python structure.

This setup is the same as what Hatchling does:

• https://github.com/pypa/hatch/blob/hatchling-v1.24.2/backend/scripts/update_licenses.py
• https://github.com/pypa/hatch/blob/hatchling-v1.24.2/backend/src/hatchling/licenses/supported.py

The parser itself and what the API should look like will be discussed in a separate PR.

The file may be generated by running: invoke licenses.update

[befeleme]

I've got one suggestion: The PEP specifies that the supported SPDX license list is version 3.17 or any later compatible version - should we add a check that nothing gets lost when updating to a newer list version?

The code looks straightforward to me and it works fine.
I'd suggest adding print(f"File written: {SPDX_LICENSES}") at the end of tasks/licenses.py - I know the path is stored somewhere else, but seeing it printed out would save me a few clicks.

I wonder whether this should be a job running on every PR or only a part of the build path?

Side note: the other task, pep440, doesn't seem to work anymore. I'll open an issue.

[ofek]

Superseded by #828