If one binding has already been imported, then qtpy should just use it #138

Consider, for example, a system where solely pyqt4 and pyqt5 are installed. If one does

import PyQt5.QtGui
import qtpy

this fails with

qtpy.PythonQtError: No Qt bindings could be found

(even though obviously there is a Qt binding available) because PyQt4.QtGui cannot be imported at all after PyQt5.QtGui has already been imported.

Fortunately there is an easy solution, which is to check, before doing anything else, whether PyQt4/5/PySide/2 are already present in sys.modules, and, if one of them is, just use it.

Real use case:
matplotlib (which can use any of the bindings) is considering switching from using its own qt shim to qtpy. However, it is not rare that one user specifies a given binding to use (via the matplotlibrc backend param), but sometimes wants to use some GUI program that is explicitly written (solely) against PyQt5. Then starting the GUI will import PyQt5 and matplotlib would fail to initialize its backend if it did not first check whether a given binding had already been forced for the process.