buildPython*: add updateScript to passthru

All Python packages now have an updateScript. The script calls
`update-python-libraries` and passes it the position of the derivation
expression obtained using `meta.position`. This works fine in case a Nix
expression represents only a single derivation. If there are more in it,
`update-python-libraries` will fail.

pkgs/development/interpreters/python/update-python-libraries/default.nix

@@ -0,0 +1,12 @@
+{ python3, runCommand, git }:
+
+runCommand "update-python-libraries" {
+  buildInputs = [
+    (python3.withPackages(ps: with ps; [ packaging requests toolz ]))
+    git
+  ];
+} ''
+  cp ${./update-python-libraries.py} $out
+  patchShebangs $out
+  substituteInPlace $out --replace 'GIT = "git"' 'GIT = "${git}/bin/git"'
+''