Scan results don't provide a consolidated_license_expression even when there is a LICENSE file

A recent scan of `ptyprocess-0.6.0.tar.gz` from
https://files.pythonhosted.org/packages/7d/2d/e4b8733cf79b7309d84c9081a4ab558c89d8c89da5961bf4ddb050ca1ce0/ptyprocess-0.6.0.tar.gz
on the page
https://pypi.org/project/ptyprocess/#files

does not return anything in the field consolidated_license_expression (or core_license_expression) even though there is a clearly detected license of `isc` in the LICENSE file that is in that package. This is unfortunate because it requires one to make a manual inspection of the scan results and the package itself to determine that `isc` is indeed the primary license, making it hard for an automated process to take advantage of the license data that scancode actually detected.

[ptyprocess-0.6.0.tar.gz_scan.json.zip](https://github.com/nexB/scancode-toolkit/files/4827728/ptyprocess-0.6.0.tar.gz_scan.json.zip)