No completion for pipx, which is using argcomplete

pipx uses argcomplete.

On Linux (Debian 11), I install pipx, xonsh and xontrib-argcomplete with

```sh
python3 -m pip install pipx
python3 -m pipx ensurepath
```

In a new terminal, I can setup argcomplete for Bash and I get autocompletion for Bash. Then,

```
pipx install xonsh
xonsh
xpip install xontrib-argcomplete
echo 'xontrib load argcomplete' >> ~/.xonshrc
```

In a new terminal, I don't get autocompletion for pipx in xonsh.