Remove QApplication.instance() deleteLater call

the test_signalproxy.py had a fixture for the QApplication instance,
only problem is at the end of each use of the fixture, it would mark
the application instance for deletion, which is most definitely not what
we want

tests/test_signalproxy.py

@@ -1,7 +1,7 @@
 import sys
 import pytest
 
-from pyqtgraph.Qt import QtCore, QtGui, QT_LIB, PYSIDE
+from pyqtgraph.Qt import QtCore, QtGui, QT_LIB, mkQApp
 from pyqtgraph import SignalProxy
 
 
@@ -24,13 +24,11 @@ class Receiver(QtCore.QObject):
 
 @pytest.fixture
 def qapp():
-    app = QtGui.QApplication.instance()
+    app = mkQApp()
     if app is None:
         app = QtGui.QApplication(sys.argv)
     yield app
-
     app.processEvents(QtCore.QEventLoop.AllEvents, 100)
-    app.deleteLater()
 
 
 def test_signal_proxy_slot(qapp):
@@ -50,13 +48,8 @@ def test_signal_proxy_slot(qapp):
     qapp.processEvents(QtCore.QEventLoop.AllEvents, 10)
 
     assert receiver.counter > 0
-    del proxy
-    del sender
-    del receiver
 
 
-@pytest.mark.skipif(QT_LIB == PYSIDE and sys.version_info < (2, 8),
-                    reason="Crashing on PySide and Python 2.7")
 def test_signal_proxy_disconnect_slot(qapp):
     """Test the disconnect of SignalProxy with `signal` and `slot`"""
     sender = Sender(parent=qapp)
@@ -80,10 +73,6 @@ def test_signal_proxy_disconnect_slot(qapp):
 
     assert receiver.counter == 0
 
-    del proxy
-    del sender
-    del receiver
-
 
 def test_signal_proxy_no_slot_start(qapp):
     """Test the connect mode of SignalProxy without slot at start`"""
@@ -113,10 +102,6 @@ def test_signal_proxy_no_slot_start(qapp):
     with pytest.raises(AssertionError):
         proxy.connectSlot(receiver.slotReceive)
 
-    del proxy
-    del sender
-    del receiver
-
 
 def test_signal_proxy_slot_block(qapp):
     """Test the block mode of SignalProxy with `signal` and `slot`"""
@@ -144,7 +129,3 @@ def test_signal_proxy_slot_block(qapp):
     qapp.processEvents(QtCore.QEventLoop.AllEvents, 10)
 
     assert receiver.counter > 0
-
-    del proxy
-    del sender
-    del receiver