Enable `stdenv` customization through `(buildPython*.override { inherit stdenv; })` (#271762)

  with the `pipInstallHook`.

- `unittestCheckHook` will run tests with `python -m unittest discover`. See [example usage](#using-unittestcheckhook).

#### Overriding build helpers {#overriding-python-build-helpers}

Like many of the build helpers provided by Nixpkgs, Python build helpers typically provide a `<function>.override` attribute.

It works like [`<pkg>.override`](#sec-pkg-override), and can be used to override the dependencies of each build helper.

This allows specifying the stdenv to be used by `buildPythonPackage` or `buildPythonApplication`. The default (`python.stdenv`) can be overridden as follows:

```nix

buildPythonPackage.override { stdenv = customStdenv; } {

  # package attrs...

}

```

## User Guide {#user-guide}

### Using Python {#using-python}

  "buildpythonpackage-parameters": [

    "index.html#buildpythonpackage-parameters"

  ],

  "overriding-python-build-helpers": [

    "index.html#overriding-python-build-helpers"

  ],

  "overriding-python-packages": [

    "index.html#overriding-python-packages"

  ],

- `emacs` now disables the GC mark trace buffer by default. This improves GC performance by 5%, but can make GC issues harder to debug. This is configurable with `withGcMarkTrace`.

- Passing `stdenv` to `buildPythonPackage` or `buildPythonApplication` has been deprecated and will trigger an error in a future release.

  Instead, you should _override_ the python build helper, e.g., `(buildPythonPackage.override { stdenv = customStdenv; })`.

  See [](#overriding-python-build-helpers).

- `buildPythonPackage` and `buildPythonApplication` now default to `nix-update-script` as their default `updateScript`. This should improve automated updates, since nix-update is better maintained than the in-tree update script and has more robust fetcher support.

- `plasma6`: Fixed the `ksycoca` cache not being re-built when `$XDG_CACHE_HOME` is set to something that isn't `$HOME/.cache`.

  lib,

  config,

  python,

  # Allow passing in a custom stdenv to buildPython*.override

  stdenv,

  wrapPython,

  unzip,

  ensureNewerSourcesForZipFilesHook,

  doCheck ? true,

  # Allow passing in a custom stdenv to buildPython*

  stdenv ? python.stdenv,

  ...

}@attrs:

      override = lib.mirrorFunctionArgs f.override (fdrv: makeOverridablePythonPackage (f.override fdrv));

    };

  overrideStdenvCompat =

    f:

    lib.setFunctionArgs (

      args:

      if !(lib.isFunction args) && (args ? stdenv) then

        lib.warnIf (lib.oldestSupportedReleaseIsAtLeast 2511) ''

          Passing `stdenv` directly to `buildPythonPackage` or `buildPythonApplication` is deprecated. You should use their `.override` function instead, e.g:

            buildPythonPackage.override { stdenv = customStdenv; } { }

        '' (f.override { stdenv = args.stdenv; } args)

      else

        f args

    ) (removeAttrs (lib.functionArgs f) [ "stdenv" ])

    // {

      # Intentionally drop the effect of overrideStdenvCompat when calling `buildPython*.override`.

      inherit (f) override;

    };

  mkPythonDerivation =

    if python.isPy3k then ./mk-python-derivation.nix else ./python2/mk-python-derivation.nix;

  buildPythonPackage = makeOverridablePythonPackage (

    overrideStdenvCompat (

      callPackage mkPythonDerivation {

        inherit namePrefix; # We want Python libraries to be named like e.g. "python3.6-${name}"

        inherit toPythonModule; # Libraries provide modules

        inherit (python) stdenv;

      }

    )

  );

  buildPythonApplication = makeOverridablePythonPackage (

    overrideStdenvCompat (

      callPackage mkPythonDerivation {

        namePrefix = ""; # Python applications should not have any prefix

        toPythonModule = x: x; # Application does not provide modules.

        inherit (python) stdenv;

      }

    )

  );

  # Check whether a derivation provides a Python module.