PEP 639 license identifier in pyproject.toml

[davetapley]

Per: https://packaging.python.org/en/latest/guides/licensing-examples-and-user-scenarios

For: https://pypi.org/project/licensecheck/ which currently shows:

┏━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━
┃ Compatible ┃ Package                   ┃ License
┡━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━
│
│ ✖          │ dataretrieval             │ LICENSE

[davetapley]

Wow validate-pyproject is fussy, not CC0 1.0 Universal, not CC0 1.0, but exactly CC0-1.0 🤪

[davetapley]

Closing for different downstream branch: #187