Add copyright/spdx license information header to all python files

[terriko]

This adds the recommended copyright/spdx information to all python files in cve-bin-tool. (This is in preparation for merging #1091 once I get approval from our licensing team; they recommend we display any additional licenses at the top below our SPDX info, but ours was missing, so I'm fixing that first.)

For the record, I used the following bash script to do this:

for a in `find . -name "*.py"`; do echo "===" $a; cat spdx_header.txt > /tmp/spdx; cat $a >> /tmp/spdx; cp /tmp/spdx $a; done

And then used a variant of that to git add all the files once I'd checked them.

[terriko]

Pinging @pdxjohnny for review. Not that there's code to review here, but I wouldn't mind a second opinion on whether we should remove the shebang (#!/usr/bin/python) lines from the few files where we have them, or possibly move the shebang to occur before the license info. I don't think it has much functional effect since most people would never try to run those files separately so it's basically a comment in most contexts, but it seems inelegant.

[terriko]

No comment from @pdxjohnny but I think I'm going to get this merged as-is since the licensing folk recommended it, and we'll decide about the #!/usr/bin/python3 lines later.

[Molkree]

Any decision on shebang lines? I see there are 5 files left with them.