bpo-43207: InspectLoader.is_package is not an abstract method (GH-24517)

Making the description of `InspectLoader.is_package` aligned with the current implementation.

Automerge-Triggered-By: GH:jaraco
(cherry picked from commit 8b9310d)

Co-authored-by: Junnosuke Kuroda <[email protected]>

Author: miss-islington

Doc/library/importlib.rst

@@ -622,7 +622,7 @@ ABC hierarchy::
 
     .. method:: is_package(fullname)
 
-        An abstract method to return a true value if the module is a package, a
+        An optional method to return a true value if the module is a package, a
         false value otherwise. :exc:`ImportError` is raised if the
         :term:`loader` cannot find the module.