pythonPackages.toPythonModule: also support overridePythonAttrs.

The `overridePythonAttrs` method is for overriding `buildPython*`
builders. Packages that use `toPythonModule` directly are typically
bindings build not using `buildPython*`.

This commit allows also those packages to be overridden with
`overridePythonAttrs`, insteading actually invoking `overrideAttrs`.

The motivation for this is to offer a common user interface for users of
the Python packages set. This way they only need to know about
`overridePythonAttrs` when overriding the builder, and `override` when
overriding the `callPackage` call.

Related RFC NixOS/rfcs#67.

pkgs/top-level/python-packages.nix

@@ -42,7 +42,7 @@ let
       }
       else ff;
 
-  buildPythonPackage = makeOverridablePythonPackage ( makeOverridable (callPackage ../development/interpreters/python/mk-python-derivation.nix {
+  buildPythonPackage = makeOverridablePythonPackage (makeOverridable (callPackage ../development/interpreters/python/mk-python-derivation.nix {
     inherit namePrefix;     # We want Python libraries to be named like e.g. "python3.6-${name}"
     inherit toPythonModule; # Libraries provide modules
   }));
@@ -81,7 +81,9 @@ let
         pythonPath = [ ]; # Deprecated, for compatibility.
         requiredPythonModules = requiredPythonModules drv.propagatedBuildInputs;
       };
-    });
+    }) // {
+      overridePythonAttrs = drv.overrideAttrs;
+    };
 
   # Convert a Python library to an application.
   toPythonApplication = drv: