Python: add buildPythonPackage.overridePythonPackage method.

[FRidh]

This allows one to always override the call to buildPythonPackage.

In the following example we create an environment where we have the blaze package using an older version of pandas. We override first the Python interpreter and pass packageOverrides which contains the overrides for packages in
the package set.

with import <nixpkgs> {};

(let
  python = let
    packageOverrides = self: super: {
      pandas = super.pandas.overridePythonPackage(old: rec {
        version = "0.19.1";
        name = "pandas-${version}";
        src =  super.fetchPypi {
          pname = "pandas";
          inherit version;
          sha256 = "08blshqj9zj1wyjhhw3kl2vas75vhhicvv72flvf1z3jvapgw295";
        };
      });
    };
  in pkgs.python35.override {inherit packageOverrides;};

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

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

---

[mention-bot]

@FRidh, thanks for your PR! By analyzing the history of the files in this pull request, we identified @alexeymuranov, @bennofs, @Azulinho and @FRidh to be potential reviewers.

[FRidh]

This is a follow-up of #24155.
The previous PR I closed because I thought it wasn't working anymore. Maybe I did something wrong then because it does seem to work now.

[FRidh]

I've pushed 345b35c to master.

[bennofs]

@FRidh sorry that I didn't comment earlier, but wouldn't it be better to name this function overrideAttrs, for consistency with stdenv?

[FRidh]

@bennofs I thought it would be good not to shadow any of the current names. overrideAttrs can still be used, although you likely wouldn't want to. Because of that, indeed, we might as well use overrideAttrs. I would prefer some more feedback before using that name.

[bennofs]

@FRidh right, didn't think of the shadowing when I wrote that comment. But perhaps then, overridePythonAttrs would fit the existing naming scheme better?

[FRidh]

@bennofs I agree
abdb58e