PEP 639 license identifier

[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

[thodson-usgs]

Fixed by #226 (merged). The pyproject.toml now uses license = "CC0-1.0" and license-files = ["LICENSE.md"] per PEP 639.