The `./tools` dir was not part of the LINTABLES in the `tox.ini`
which meant that pep8/pylint etc checks were not run on the tools
there.
This commit adds it and fixes the issues that `make lint` found.
This pulls the list of snapshots from the rpmrepo API, greps the
codebase for all uses of rpmrepo.osbuild.org that look like a snapshot
name, and then checks to make sure they are still valid.
This is the same script as osbuild-composer, except that it also skips
checking ./test/data/stages/ which has rpm urls that look like snapshot
urls.