graph-tool: 2.16 -> 2.26

- add ncurses: configure links against ncurses and fails otherwise
    configure: error: Could not link test program to Python.
    https://travis-ci.org/NixOS/nixpkgs/builds/48759067
    The given hint (Maybe the main Python library has been installed
    in some non-standard library path) is misleading.
    The config.log reveals that the failure is due to missing ncurses link option
- with-boost-libdir is need to find Boost::IOStreams/regex/etc.
- expat/cgal are detected in /usr/lib when not specified explicitly
- boost > boost159 is needed to have -lboost_python3 (and -lboost_python)

- set pythonModule = Python;
  => inorder to be used in python.buildEnv { extraLibs = [..]; }

tested on MacOSX and in a linux Docker container with:
> nix-shell -I nixpkgs=. -p python2.pkgs.graph-tool
> nix-shell -I nixpkgs=. -p python3.pkgs.graph-tool