Remove deprecated is_pyqt5 function.

Author: QuLogic

doc/api/next_api_changes/removals/19894-ES.rst

@@ -3,3 +3,8 @@ Qt4-based backends
 The qt4agg and qt4cairo backends have been removed. Qt4 has reached its
 end-of-life in 2015 and there are no releases of either PyQt4 or PySide for
 recent versions of Python. Please use one of the Qt5 or Qt6 backends.
+
+``qt_compat.is_pyqt5``
+~~~~~~~~~~~~~~~~~~~~~~
+This function has been removed due to the release of PyQt6. The Qt version can
+be checked using ``QtCore.qVersion()``.

lib/matplotlib/backends/qt_compat.py

@@ -17,7 +17,6 @@
 import sys
 
 import matplotlib as mpl
-from matplotlib import _api
 
 
 QT_API_PYQT5 = "PyQt5"
@@ -80,10 +79,6 @@ def _isdeleted(obj): return not shiboken2.isValid(obj)
         raise ValueError("Unexpected value for the 'backend.qt5' rcparam")
     _getSaveFileName = QtWidgets.QFileDialog.getSaveFileName
 
-    @_api.deprecated("3.3", alternative="QtCore.qVersion()")
-    def is_pyqt5():
-        return True
-
 
 if QT_API in [QT_API_PYQT5, QT_API_PYSIDE2]:
     _setup_pyqt5()