PEP 639 license identifier

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
```