Update test/README.md so it shows the correct versions of Python

[terriko]

https://github.com/intel/cve-bin-tool/blob/main/test/README.md currently says that we're testing Python 3.6 & 3.7 in CI. That's not true any more, we should update it, and update the virtualenv instructions so they no longer use 3.6 (maybe 3.8 & 3.9 would be better now?)

We're actually testing 3.7-3.10 at the moment, but you might want to direct people to check Github Actions to see what CI is currently running rather than listing versions everywhere.