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)

pkgs/development/python-modules/pipx/default.nix

@@ -4,6 +4,7 @@
 , fetchFromGitHub
 , hatchling
 , hatch-vcs
+, installShellFiles
 , packaging
 , platformdirs
 , pytestCheckHook
@@ -41,6 +42,10 @@ buildPythonPackage rec {
     tomli
   ];
 
+  nativeBuildInputs = [
+      installShellFiles
+  ];
+
   nativeCheckInputs = [
     pytestCheckHook
     git
@@ -79,6 +84,13 @@ buildPythonPackage rec {
     "test_skip_maintenance"
   ];
 
+  postInstall =  ''
+    installShellCompletion --cmd pipx \
+      --bash <(${argcomplete}/bin/register-python-argcomplete pipx --shell bash) \
+      --zsh <(${argcomplete}/bin/register-python-argcomplete pipx --shell zsh) \
+      --fish <(${argcomplete}/bin/register-python-argcomplete pipx --shell fish)
+  '';
+
   meta = with lib; {
     description = "Install and run Python applications in isolated environments";
     mainProgram = "pipx";