Migrate from Sourceforge to Github

I've written a Python script to migrate issues. It's at https://github.com/ttencate/sf2github.

Beware: Sunday afternoon software. Use at your own risk, etc. etc. Pull requests welcome!

For SVN to GitHub part, this is now the easiest way: https://help.github.com/en/github/importing-your-projects-to-github/importing-a-repository-with-github-importer

But it doesn't import issues.