Mercurial startup time is already 45.8x slower than Git whereas tested Mercurial runs on Python 2.7.12. Now try to sell Python 3 to Mercurial developers, with a startup time 2x - 3x slower...
So please continue efforts for make Python startup even faster to beat all other programming languages, and finally convince Mercurial to upgrade ;-)
https://www.mercurial-scm.org/wiki/Python3Nevertheless, I can't really be annoyed or upset at them moving slowly to adopt Python 3, as Matt's objections were entirely legitimate.