Feature Request: Support publishing to testPyPI without manual edit to .pypirc file

[Mariatta]

Follow up from https://github.com/takluyver/flit/issues/122

Members of Vancouver PyLadies are interested to learn how to publish a package to PyPI, but they are not package maintainers. I'm thinking to introduce flit to our members during an upcoming workshop. I'm thinking that they should publish to testPyPI instead of to the production PyPI.

I now know how to publish to testPyPI using flit, but if this can be done without them needing to modify ~/.pypirc manually, then it would make things simpler.

Thanks for the great tool, and I hope you will consider this feature request 🙏 😄