Fix setupext.py for the case where both PyQt4 and PyQt5 are installed

If both PyQt4 and PyQt5 are installed in the same Python the setup check
for the modules will cause a RuntimeError as they are both imported.

The Gtk module uses a multiprocessing pool to perform the imports, and this
approach is used here. However, if we import PyQt4 normally, then PyQt5
using multiprocessing the import is incomplete (no version information).
Instead, we default to multiprocessing for both PyQt4 and PyQt5 and only
fall to normal imports if that is not available.

In the case of a RuntimeError on a normal-style import the error is raised
as `CheckFailed("Could not import: are PyQt4 & PyQt5 both installed?")` to
give a hint as to the problem.