TST: Prepare for pytest 9

The current version of pytest is warning that using `importorskip` to
catch `ImportError` will start being ignored (and thus raising) with
pytest 9.

Fortunately, in all cases, we don't need these calls, as they are:

- already checked for `ImportError` at the top-level of the file
- already checked by the backend switcher
- not actually possible to fail importing

lib/matplotlib/tests/test_backend_bases.py

@@ -1,3 +1,5 @@
+import importlib
+
 from matplotlib import path, transforms
 from matplotlib.backend_bases import (
     FigureCanvasBase, KeyEvent, LocationEvent, MouseButton, MouseEvent,
@@ -325,9 +327,7 @@ def test_toolbar_home_restores_autoscale():
 def test_draw(backend):
     from matplotlib.figure import Figure
     from matplotlib.backends.backend_agg import FigureCanvas
-    test_backend = pytest.importorskip(
-        f'matplotlib.backends.backend_{backend}'
-    )
+    test_backend = importlib.import_module(f'matplotlib.backends.backend_{backend}')
     TestCanvas = test_backend.FigureCanvas
     fig_test = Figure(constrained_layout=True)
     TestCanvas(fig_test)

lib/matplotlib/tests/test_backend_gtk3.py

@@ -3,9 +3,6 @@
 import pytest
 
 
-pytest.importorskip("matplotlib.backends.backend_gtk3agg")
-
-
 @pytest.mark.backend("gtk3agg", skip_on_importerror=True)
 def test_correct_key():
     pytest.xfail("test_widget_send_event is not triggering key_press_event")

lib/matplotlib/tests/test_backend_qt.py

@@ -26,9 +26,7 @@
 
 @pytest.fixture
 def qt_core(request):
-    qt_compat = pytest.importorskip('matplotlib.backends.qt_compat')
-    QtCore = qt_compat.QtCore
-
+    from matplotlib.backends.qt_compat import QtCore
     return QtCore