Python: add buildPythonPackage.overridePythonPackage method.

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.python3.override {inherit packageOverrides;};

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

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