mininet: init at 2.3.0d4 (#41261)

Mininet (https://github.com/mininet/mininet) is a popular network emulator that
glues several components such as network namespaces, traffic control
commands into a set of python bindings. It is then "easy" to describe a
topology and run experiments on it.

pkgs/top-level/python-packages.nix

@@ -422,6 +422,8 @@ in {
 
   monty = callPackage ../development/python-modules/monty { };
 
+  mininet-python = (toPythonModule (pkgs.mininet.override{ inherit python; })).py;
+
   mpi4py = callPackage ../development/python-modules/mpi4py {
     mpi = pkgs.openmpi;
   };