drop usage of QDesktopWidget

this drops support for Qt4

pyqtgraph/exporters/PrintExporter.py

@@ -45,11 +45,7 @@ class PrintExporter(Exporter):
         #res = printer.resolution()
         sr = self.getSourceRect()
         #res = sr.width() * .4 / (self.params['width'] * 100 / 2.54)
-        try:
-            res = QtGui.QDesktopWidget().physicalDpiX()
-        except AttributeError:
-            # This is available since Qt 5
-            res = QtGui.QGuiApplication.primaryScreen().physicalDotsPerInchX()
+        res = QtGui.QGuiApplication.primaryScreen().physicalDotsPerInchX()
         printer.setResolution(res)
         rect = printer.pageRect()
         center = rect.center()

pyqtgraph/exporters/SVGExporter.py

@@ -195,11 +195,7 @@ def _generateItemSvg(item, nodes=None, root=None, options={}):
         buf = QtCore.QBuffer(arr)
         svg = QtSvg.QSvgGenerator()
         svg.setOutputDevice(buf)
-        try:
-            dpi = QtGui.QDesktopWidget().logicalDpiX()
-        except AttributeError:
-            # This is available since Qt 5
-            dpi = QtGui.QGuiApplication.primaryScreen().logicalDotsPerInchX()
+        dpi = QtGui.QGuiApplication.primaryScreen().logicalDotsPerInchX()
         svg.setResolution(dpi)
 
         p = QtGui.QPainter()