### Summary
Adds a CI check to ensure that packages added as dependencies are
appropriately licensed. All of the `.txt` files in the `requirements`
directory are checked with the exception of:
- `constraints.txt`, since those are not installed and are instead
conditions on the other dependency files
- `dev.txt`, since those are for local development and not shipped as
part of the `unstructured` package
- `extra-pdf-image.txt` - the `extra-pdf-image.in` since checking
`extra-pdf-image.txt` pulls in NVIDIA GPU related packages with an
`Other/Proprietary` license type, and there's not a good way to exclude
those without adding `Other/Proprietary` to the allowed licenses list.
### Testing
The new `check-licenses` job should pass in CI.
