Tests fail when both PySide2 and PyQt5 are installed in the test env

[jni]

Describe the bug

If I have both PySide2 and PyQt5 in the environment, I get the following test failure:

 $ pytest tests/test_magicgui.py::test_auto_call
=============================== test session starts ===============================
platform linux -- Python 3.8.2, pytest-6.2.2, py-1.10.0, pluggy-0.13.1 -- /home/jni/miniconda3/envs/all/bin/python
cachedir: .pytest_cache
PySide2 5.14.2.1 -- Qt runtime 5.14.2 -- Qt compiled 5.14.2
rootdir: /home/jni/projects/magicgui, configfile: setup.cfg
plugins: mypy-plugins-1.6.1, napari-plugin-engine-0.1.9, timeout-1.3.4, httpserver-0.3.4, ordering-0.6, qt-3.3.0, cov-2.8.1, napari-0.4.4.dev89+ge33c2fd1
collected 1 item                                                                  

tests/test_magicgui.py::test_auto_call FAILED                               [100%]

==================================== FAILURES =====================================
_________________________________ test_auto_call __________________________________

qtbot = <pytestqt.qtbot.QtBot object at 0x7f0d8dce2940>
magic_func = <FunctionGui func(a: str = 'works', b: int = 6, c=7.1) -> str>

    def test_auto_call(qtbot, magic_func):
        """Test that changing a parameter calls the function."""
    
        # TODO: remove qtbot requirement so we can test other backends eventually.
    
        # changing the widget parameter calls the function
        with qtbot.waitSignal(magic_func.called, timeout=1000):
            magic_func.b.value = 6
    
        # changing the gui calls the function
        with qtbot.waitSignal(magic_func.called, timeout=1000):
>           qtbot.keyClick(magic_func.a.native, Qt.Key_A, Qt.ControlModifier)

tests/test_magicgui.py:131: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

args = (<PyQt5.QtWidgets.QLineEdit object at 0x7f0d917079d0>, 65, 67108864)
kwargs = {}

    def result(*args, **kwargs):
>       return qtest_method(*args, **kwargs)
E       TypeError: 'PySide2.QtTest.QTest.keyClick' called with wrong argument types:
E         PySide2.QtTest.QTest.keyClick(QLineEdit, Key, KeyboardModifier)
E       Supported signatures:
E         PySide2.QtTest.QTest.keyClick(PySide2.QtWidgets.QWidget, PySide2.QtCore.Qt.Key, PySide2.QtCore.Qt.KeyboardModifiers = PySide2.QtCore.Qt.KeyboardModifier.NoModifier, int = -1)
E         PySide2.QtTest.QTest.keyClick(PySide2.QtWidgets.QWidget, typing.Char, PySide2.QtCore.Qt.KeyboardModifiers = PySide2.QtCore.Qt.KeyboardModifier.NoModifier, int = -1)
E         PySide2.QtTest.QTest.keyClick(PySide2.QtGui.QWindow, PySide2.QtCore.Qt.Key, PySide2.QtCore.Qt.KeyboardModifiers = PySide2.QtCore.Qt.KeyboardModifier.NoModifier, int = -1)
E         PySide2.QtTest.QTest.keyClick(PySide2.QtGui.QWindow, typing.Char, PySide2.QtCore.Qt.KeyboardModifiers = PySide2.QtCore.Qt.KeyboardModifier.NoModifier, int = -1)

../../miniconda3/envs/all/lib/python3.8/site-packages/pytestqt/qtbot.py:626: TypeError
============================= short test summary info =============================
FAILED tests/test_magicgui.py::test_auto_call - TypeError: 'PySide2.QtTest.QTest...
================================ 1 failed in 0.22s ================================

Using QT_API=PySide2 pytest tests/test_magicgui.py::test_auto_call resolves the problem, but QT_API=PyQt5 pytest tests/test_magicgui.py::test_auto_call does not. So it seems pytest-qtbot is choosing PySide2 and ignoring the QT_API variable.

Expected behavior
Failure could be "expected" and marked as xfail, or magicgui could match pytest-qtbot during testing.

Environment (please complete the following information):

• OS: mac, Linux
• backend: Both!
• magicgui version 61d8f4f