pipx: add shell completions
Install shell completions with `pipx` for `bash`, `zsh` and `fish`.
Apart from convenience, this reduces confusion. `pipx completions` is
not quite accurate when `pipx` is installed with Nix, since you need to
do `nix shell nixpkgs#python311Packages.argcomplete
register-python-argcomplete`. It is possible to also put it in the path,
as suggested in
https://discourse.nixos.org/t/python-how-to-expose-a-dependencys-bin/283,
but that would be a separate PR.
Fixes https://github.com/NixOS/nixpkgs/issues/299845
Cc: [@natsukium](https://github.com/natsukium),
[@mweinelt](https://github.com/mweinelt),
[@yshym](https://github.com/yshym) (same people I Cc-ed in the issue)
Ilya Grigoriev
56be20aa49