ci: Remove extra conditions for optional GUI dependencies

We aren't testing on macOS 10.12 any more, but macos-latest, which is
now macOS 12. Wheels for Python 3.12 should be available everywhere by
now as well.

Only PySide2 remains conditional, as it is deprecated, but provides
universal wheels that install but don't work on Python 3.12.

Author: QuLogic

.github/workflows/tests.yml

@@ -232,7 +232,6 @@ jobs:
           # (sometimes, the install appears to be successful but shared
           # libraries cannot be loaded at runtime, so an actual import is a
           # better check).
-          # PyGObject, pycairo, and cariocffi do not install on macOS 10.12.
           python -m pip install --upgrade pycairo 'cairocffi>=0.8' PyGObject &&
             (
               python -c 'import gi; gi.require_version("Gtk", "4.0"); from gi.repository import Gtk' &&
@@ -242,43 +241,35 @@ jobs:
               echo 'PyGObject 3 is available' || echo 'PyGObject 3 is not available'
             )
 
-          # There are no functioning wheels available for macOS 10.12 (as of
-          # Sept 2020) for either pyqt5 (there are only wheels for 10.13+) or
-          # pyside2 (the latest version (5.13.2) with 10.12 wheels has a
-          # fatal to us bug, it was fixed in 5.14.0 which has 10.13 wheels)
           python -mpip install --upgrade pyqt5${{ matrix.pyqt5-ver }} &&
             python -c 'import PyQt5.QtCore' &&
             echo 'PyQt5 is available' ||
             echo 'PyQt5 is not available'
-          if [[ "${{ runner.os }}" != 'macOS'
+          # Even though PySide2 wheels can be installed on Python 3.12, they are broken and since PySide2 is
+          # deprecated, they are unlikely to be fixed. For the same deprecation reason, there are no wheels
+          # on M1 macOS, so don't bother there either.
+          if [[ "${{ matrix.os }}" != 'macos-14'
                 && "${{ matrix.python-version }}" != '3.12' ]]; then
             python -mpip install --upgrade pyside2${{ matrix.pyside2-ver }} &&
               python -c 'import PySide2.QtCore' &&
               echo 'PySide2 is available' ||
               echo 'PySide2 is not available'
           fi
-          if [[ "${{ runner.os }}" != 'macOS' ]]; then
-            python -mpip install --upgrade pyqt6${{ matrix.pyqt6-ver }} &&
-              python -c 'import PyQt6.QtCore' &&
-              echo 'PyQt6 is available' ||
-              echo 'PyQt6 is not available'
-          fi
-          if [[ "${{ runner.os }}" != 'macOS'
-                && "${{ matrix.python-version }}" != '3.12' ]]; then
-            python -mpip install --upgrade pyside6${{ matrix.pyside6-ver }} &&
-              python -c 'import PySide6.QtCore' &&
-              echo 'PySide6 is available' ||
-              echo 'PySide6 is not available'
-          fi
+          python -mpip install --upgrade pyqt6${{ matrix.pyqt6-ver }} &&
+            python -c 'import PyQt6.QtCore' &&
+            echo 'PyQt6 is available' ||
+            echo 'PyQt6 is not available'
+          python -mpip install --upgrade pyside6${{ matrix.pyside6-ver }} &&
+            python -c 'import PySide6.QtCore' &&
+            echo 'PySide6 is available' ||
+            echo 'PySide6 is not available'
 
-          if [[ "${{ matrix.python-version }}" != '3.12' ]]; then
-            python -mpip install --upgrade \
-              -f "https://extras.wxpython.org/wxPython4/extras/linux/gtk3/${{ matrix.os }}" \
-              wxPython &&
-              python -c 'import wx' &&
-              echo 'wxPython is available' ||
-              echo 'wxPython is not available'
-          fi
+          python -mpip install --upgrade \
+            -f "https://extras.wxpython.org/wxPython4/extras/linux/gtk3/${{ matrix.os }}" \
+            wxPython &&
+            python -c 'import wx' &&
+            echo 'wxPython is available' ||
+            echo 'wxPython is not available'
 
       - name: Install the nightly dependencies
         # Only install the nightly dependencies during the scheduled event

lib/matplotlib/tests/test_backends_interactive.py

@@ -771,6 +771,13 @@ def test_other_signal_before_sigint(env, target, kwargs, request):
         pytest.skip("SIGINT currently only tested on qt and macosx")
     if backend == "macosx":
         request.node.add_marker(pytest.mark.xfail(reason="macosx backend is buggy"))
+    if sys.platform == "darwin" and target == "show":
+        # We've not previously had these toolkits installed on CI, and so were never
+        # aware that this was crashing. However, we've had little luck reproducing it
+        # locally, so mark it xfail for now. For more information, see
+        # https://github.com/matplotlib/matplotlib/issues/27984
+        request.node.add_marker(
+            pytest.mark.xfail(reason="Qt backend is buggy on macOS"))
     proc = _WaitForStringPopen(
         [sys.executable, "-c",
          inspect.getsource(_test_other_signal_before_sigint_impl) +