pipx-in-pipx, all you need to do is install :
$ pip install pipx-in-pipx
But wait! You say. Didn’t you just say that we shouldn’t install things to system Python?
pipx-in-pipx actually does is slightly (but only slightly) evil.
Rather than actually installing anything when you run “install”,
pipx-in-pipx instead builds a temporary virtual environment,
installs pipx there,
and then uses that pipx to install pipx in your user local space,
just like any other pipx-installed tool.