doc/python: document fixed-point arguments (#271387) (#477149)

* `setupPyGlobalFlags ? []`: List of flags passed to `setup.py` command.

* `setupPyBuildFlags ? []`: List of flags passed to `setup.py build_ext` command.

##### Using fixed-point arguments {#buildpythonpackage-fixed-point-arguments}

Both `buildPythonPackage` and `buildPythonApplication` support [fixed-point arguments](#chap-build-helpers-finalAttrs), similar to `stdenv.mkDerivation`.

This allows you to reference the final attributes of the derivation.

Instead of using `rec`:

```nix

buildPythonPackage rec {

  pname = "pyspread";

  version = "2.4";

  src = fetchPypi {

    inherit pname version;

    hash = "sha256-...";

  };

}

```

You can use the `finalAttrs` pattern:

```nix

buildPythonPackage (finalAttrs: {

  pname = "pyspread";

  version = "2.4";

  src = fetchPypi {

    pname = "pyspread";

    inherit (finalAttrs) version;

    hash = "sha256-...";

  };

})

```

See the [general documentation on fixed-point arguments](#chap-build-helpers-finalAttrs) for more details on the benefits of this pattern.

::: {.note}

Some `buildPythonPackage`/`buildPythonApplication` arguments are passed down indirectly to `stdenv.mkDerivation` via `passthru`.

Therefore the final state of these attributes can be accessed via `finalAttrs.passthru.${name}`.

[`<pkg>.overrideAttrs`](#sec-pkg-overrideAttrs) can override them using the `passthru = prevAttrs.passthru // { foo = "bar"; }` pattern.

Such arguments include:

- `disabled`

- `pyproject`

- `format`

- `build-system`

- `dependencies`

- `optional-dependencies`

<!--

TODO(@doronbehar): When `.overridePythonAttrs` will be removed, the above text might need to be revised. See:

- https://github.com/NixOS/nixpkgs/pull/379637

- https://github.com/NixOS/nixpkgs/pull/469804

-->

:::

The [`stdenv.mkDerivation`](#sec-using-stdenv) function accepts various parameters for describing

build inputs (see "Specifying dependencies"). The following are of special

interest for Python packages, either because these are primarily used, or

```nix

with import <nixpkgs> { };

(

  let

    python =

let

  python = pkgs.python3.override {

    packageOverrides = self: super: {

          pandas = super.pandas.overridePythonAttrs (old: rec {

      pandas = super.pandas.overridePythonAttrs (

        finalAttrs: prevAttrs: {

          version = "0.19.1";

          src = fetchPypi {

            pname = "pandas";

              inherit version;

            inherit (finalAttrs) version;

            hash = "sha256-JQn+rtpy/OA2deLszSKEuxyttqBzcAil50H+JDHUdCE=";

          };

          });

        }

      );

    };

      in

      pkgs.python3.override {

        inherit packageOverrides;

        self = python;

  };

in

  python.withPackages (ps: [ ps.blaze ])

).env

(python.withPackages (ps: [ ps.blaze ])).env

```

The next example shows a non trivial overriding of the `blas` implementation to

  "buildpythonpackage-parameters": [

    "index.html#buildpythonpackage-parameters"

  ],

  "buildpythonpackage-fixed-point-arguments": [

    "index.html#buildpythonpackage-fixed-point-arguments"

  ],

  "overriding-python-build-helpers": [

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

  ],