Rerun TestPyQIODevice.test_qprocess if needed.

This adds a new pytest-rerunfailures dependency. For some reason, that test
sometimes fails with an empty string read, and I can't tell why.

pytest.ini

@@ -15,6 +15,7 @@ markers =
     skip: Always skipped test.
     pyqt531_or_newer: Needs PyQt 5.3.1 or newer.
     xfail_norun: xfail the test with out running it
+    flaky: Tests which are flaky and should be rerun
 qt_log_level_fail = WARNING
 qt_log_ignore =
     ^SpellCheck: .*

tests/unit/utils/test_qtutils.py

@@ -760,6 +760,7 @@ class TestPyQIODevice:
         with pytest.raises(io.UnsupportedOperation):
             pyqiodev.seek(0, whence)
 
+    @pytest.mark.flaky(reruns=1)
     def test_qprocess(self, py_proc):
         """Test PyQIODevice with a QProcess which is non-sequential.
 

tox.ini

@@ -37,6 +37,7 @@ deps =
     pytest-instafail==0.3.0
     pytest-travis-fold==1.2.0
     pytest-repeat==0.2
+    pytest-rerunfailures==1.0.1
     six==1.10.0
     termcolor==1.1.0
     vulture==0.8.1