There are other packages that depend on python3-mercurial, like debian's
mercurial-git, so we should mark ourselves as providing it.
I compared the control file we generate to the one that the debian maintainers
generate, and noticed several differences:
- the Homepage bit. I included this, because why not
- a more robust Suggests list that includes a graphical merge tool
- a more robust Breaks list
- debian's Recommends openssh-client, we only Recommends ca-certificates
- a split into mercurial and mercurial-common (and possibly others?)
- a slightly different description