pythonGH-119054: Add alt text to pathlib inheritance diagram

Doc/library/pathlib.rst

@@ -21,6 +21,15 @@ inherit from pure paths but also provide I/O operations.
 .. image:: pathlib-inheritance.png
    :align: center
    :class: invert-in-dark-mode
+   :alt: Inheritance diagram showing the classes available in pathlib. The
+         most basic class is ``PurePath``, which has three direct subclasses:
+         ``PurePosixPath``, ``PureWindowsPath``, and ``Path``. Further to
+         these four classes, there are two classes that use multiple
+         inheritance: ``PosixPath`` subclasses ``PurePosixPath`` and ``Path``,
+         and ``WindowsPath`` subclasses ``PureWindowsPath`` and ``Path``. The
+         upper part of the diagram shows the three pure classes, starting with
+         ``PurePath``, and the lower part shows the three concrete classes,
+         ending with ``PosixPath`` and ``WindowsPath``.
 
 If you've never used this module before or just aren't sure which class is
 right for your task, :class:`Path` is most likely what you need. It instantiates