Add maxlen argument to ErrorNetworkReply.readData.

This was missing before, causing a (hidden) exception with Python < 3.5, and
this with 3.5:

    TypeError: readData() takes 1 positional argument but 2 were given

    During handling of the above exception, another exception occurred:

    SystemError: PyEval_EvalFrameEx returned a result with an error set

Fixes #969.

qutebrowser/browser/network/networkreply.py

@@ -127,7 +127,7 @@ class ErrorNetworkReply(QNetworkReply):
         """We always have 0 bytes available."""
         return 0
 
-    def readData(self):
+    def readData(self, _maxlen):
         """No data available."""
         return bytes()
 

tests/unit/browser/network/test_networkreply.py

@@ -88,6 +88,6 @@ def test_error_network_reply(qtbot, req):
     assert reply.isFinished()
     assert not reply.isRunning()
     assert reply.bytesAvailable() == 0
-    assert reply.readData() == b''
+    assert reply.readData(1) == b''
     assert reply.error() == QNetworkReply.UnknownNetworkError
     assert reply.errorString() == "This is an error"