Bump PyGobject depdendency to 3.6.0 or later.

I don't really know what's needed, but 3.4.2 seems too old.

https://bugzilla.gnome.org/show_bug.cgi?id=702361
Use float.fromhex instead of eval.

eval is dangerous.
Avoid KeyError tearing down extensions.

Extensions are torn down in random order as keys returned from a python
dictionary and thus due to the dependency handling the top-level
requested module might already be torn down. Perhaps.

https://bugzilla.gnome.org/show_bug.cgi?id=702518
Update Spanish translation.