Cannot determine why consolidation returns an "unknown" in the license expressions

A recent scan of `pycparser-2.20.tar.gz` from
https://files.pythonhosted.org/packages/0f/86/e19659527668d70be91d0369aeaa055b4eb396b0f387a4f92293a20035bd/pycparser-2.20.tar.gz
on the page at
https://pypi.org/project/pycparser/#files

returns some unexpected license results that are mysterious:
```
license_expression | "unknown AND bsd-new"
-- | --
declared_license | {…}
notice_text | null
root_path | "pycparser-2.20.tar.gz-ex…pycparser-2.20/setup.py"
dependencies | []
contains_source_code | null
source_packages | []
purl | "pkg:pypi/pycparser@2.20"
repository_homepage_url | null
repository_download_url | null
api_data_url | null
identifier | "pkg_pypi_pycparser_2_20_1"
consolidated_license_expression | "bsd-new AND unknown"
consolidated_holders | []
consolidated_copyright | "Copyright (c) "
core_license_expression | "unknown AND bsd-new"
```
The scan results also return individual license keys of bsd-new, public-domain, and lgpl-2.0-plus, but there is no "unknown" license key in the rest of the scan results, so the reader of the overall scan results has no way to determine why the various license_expression fields contain "... AND unknown" or "unknown AND ...".

[pycparser-2.20.tar.gz_scan.json.zip](https://github.com/nexB/scancode-toolkit/files/4827994/pycparser-2.20.tar.gz_scan.json.zip)