gh-137586: Fix Sphinx ref.class warnings in whatsnew/3.15.rst

Use :class:`!webbrowser.MacOS` and :class:`!webbrowser.MacOSXOSAScript`
with the ! prefix to suppress cross-reference lookup for classes not yet
present in upstream.

Author: secengjeff

Doc/whatsnew/3.15.rst

@@ -1393,7 +1393,7 @@ wave
 webbrowser
 ----------
 
-* On macOS, the new :class:`~webbrowser.MacOS` class opens URLs via
+* On macOS, the new :class:`!webbrowser.MacOS` class opens URLs via
   :program:`/usr/bin/open` instead of constructing and executing AppleScript
   via :program:`osascript`. The default browser is detected from the
   LaunchServices preferences file using :mod:`plistlib`, with
@@ -1795,8 +1795,8 @@ New deprecations
 
 * :mod:`webbrowser`:
 
-  * :class:`~webbrowser.MacOSXOSAScript` is deprecated in favour of
-    :class:`~webbrowser.MacOS` and scheduled for removal in Python 3.17.
+  * :class:`!webbrowser.MacOSXOSAScript` is deprecated in favour of
+    :class:`!webbrowser.MacOS` and scheduled for removal in Python 3.17.
     (Contributed by Jeff Lyon in :gh:`137586`.)
 
 * ``__version__``