Poetry: Support mercurial for dependencies

[chrahunt]

• [x] I have searched the issues of this repo and believe that this is not a duplicate.
• [x] I have searched the documentation and believe that my question is not covered.

Feature Request

Currently, Poetry supports PyPI, custom PyPI-like repositories, and git repositories. It would be helpful to also support Mercurial to consume e.g. Pygments from the source repository.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[cognifloyd]

Yes. This is still needed.