buildPython*: bring back buildPython*.override

Fix `makeOverridablePythonPackage` in python-package-base.nix
and unshadow `buildPython*.override`.

This makes it possible to override the dependencies of buildPython*.
E.g., `buildPythonPackage.override { unzip = unzip-custom; }`
returns a derived version of `buildPythonPackage` with
the `unzip` package overridden with `unzip-custom`.

pkgs/development/interpreters/python/python-packages-base.nix

@@ -38,7 +38,12 @@ let
         }
       else
         result
-    );
+    )
+    // lib.optionalAttrs (f ? override) {
+      # Support overriding `f` itself, e.g. `buildPythonPackage.override { }`.
+      # Ensure `makeOverridablePythonPackage` is applied to the result.
+      override = lib.mirrorFunctionArgs f.override (fdrv: makeOverridablePythonPackage (f.override fdrv));
+    };
 
   mkPythonDerivation =
     if python.isPy3k then ./mk-python-derivation.nix else ./python2/mk-python-derivation.nix;