Update SPDX license list to 3.20

[pombredanne]

We have a new SPDX license list release... this is going to be special because:

• there are 40 licenses
• all are like already in ScanCode, so we have to keep the current SPDX id in "other keys" and use the new SPDX one as the official one. As likely is usually the case, SPDX has historically made little effort to reuse ScanCode license keys even when these were assigned long before and we likely have a lot of divergence there.
• SPDX texts are always problematic and subtly damaged and modified such as including top level copyright that are not part of the license, XML markup damages and the routine inclusion of instructions above and below that are not part of the license proper, therefore we can rarely reuse these as-is as they do not represent the correct original and pristine reference text. However, we may still want to keep the SPDX texts as license rules