Update python tests for Python 3

print was changed from a statement to a function in 3.0, but as simple
print calls, this syntax is compatible with both 2.x and 3.x.

Signed-off-by: Yaakov Selkowitz <yselkowitz@users.sourceforge.net>

https://bugzilla.gnome.org/show_bug.cgi?id=691268
CompletionModel: wipe out the current implementation

Keep the maximum for the new implementation.
There are also a few minor changes.
CompletionModel: separate functions into sections
CompletionModel: the new data structures
CompletionModel: implement the GtkTreeModel interface
CompletionModel: construction and destruction
CompletionModel: implement population
CompletionModel: implement get/set visible providers
CompletionModel: implement other public functions
Free return value of gtk_source_completion_model_get_providers()

In select_provider(), the variable 'providers' was declared a second
time, that's why it's renamed to 'visible_providers'.