MercurialとPython3。

https://www.mercurial-scm.org/wiki/Python3によれば、

1. Status
We have been testing by installing default mercurial on our system using Python 3. Most of the things work correctly. Things which don't work can be found at BetaBugs section below.

We are planning to mark hg 5.0 which is scheduled for May 1 as Python 3 beta release.

だそうです。