New formula for PyGobject3

Adds a new formula for PyGobject3. This formula depends on py3cairo which is also in pull request https://github.com/mxcl/homebrew/pull/23252. 

PyGobject3 compiles for both python 2 and 3 on my system, but the optional '--with-tests' freezes on the test "test_tree_view (test_overrides_gtk.TestTreeView) ..."  using 100 % CPU. This happens after passing many other tests successfully.