tox: Update pytest-qt to 2.0.0

See #1702.

Breaking Changes

With pytest-qt 2.0, we changed some defaults to values we think are much
better, however this required some backwards-incompatible changes:

- pytest-qt now defaults to using PyQt5 if PYTEST_QT_API is not set.
  Before, it preferred PySide which is using the discontinued Qt4.
- Python 3 versions prior to 3.4 are no longer supported.
- The @pytest.mark.qt_log_ignore mark now defaults to extend=True, i.e.
  extends the patterns defined in the config file rather than overriding
  them. You can pass extend=False to get the old behaviour of overriding
  the patterns.
- qtbot.waitSignal now defaults to raising=True and raises an exception
  on timeouts. You can set qt_wait_signal_raising = false in your config
  to get back the old behaviour.
- PYTEST_QT_FORCE_PYQT environment variable is no longer supported. Set
  PYTEST_QT_API to the appropriate value instead or the new qt_api
  configuration option in your pytest.ini file.

New Features

- From this version onward, pytest-qt is licensed under the MIT license.
- New qtmodeltester fixture to test QAbstractItemModel subclasses.
- waitSignal and waitSignals can receive an optional callback that can
  evaluate if the arguments of emitted signals should resume execution
  or not.
- Now which Qt binding pytest-qt will use can be configured by the
  qt_api config option.
- While pytestqt.qt_compat is an internal module and shouldn't be
  imported directly, it is known that some test suites did import it.
  This module now uses a lazy-load mechanism to load Qt classes and
  objects, so the old symbols (QtCore, QApplication, etc.) are no longer
  available from it.

Other Changes

- Exceptions caught by pytest-qt in sys.excepthook are now also printed
  to stderr, making debugging them easier from within an IDE.

misc/requirements/requirements-tests.txt

@@ -22,7 +22,7 @@ pytest-cov==2.3.0
 pytest-faulthandler==1.3.0
 pytest-instafail==0.3.0
 pytest-mock==1.1
-pytest-qt==1.11.0
+pytest-qt==2.0.0
 pytest-repeat==0.3.0
 pytest-rerunfailures==2.0.0
 pytest-travis-fold==1.2.0