hgcli uses a specific rust-cpython commit by indygreg, of which
a PR has been derived which is not merged nor released yet.
But we can't use several versions of the sys-python2.7 crate in
a single workspace: it makes for a build error. Since hgcli does not at
the time being need anything from hg-core, whereas the upcoming hg-cpython
will. So for now we're moving hgcli aside, hoping we could base all of them
on the same version of rust-cpython again in the future.