bpo-40394 - difflib.SequenceMatched.find_longest_match default args (GH-19742)

* bpo-40394 - difflib.SequenceMatched.find_longest_match default args

Added default args to find_longest_match, as well as related tests.
diff --git a/Doc/library/difflib.rst b/Doc/library/difflib.rst
index ada311b..7a898c2 100644
--- a/Doc/library/difflib.rst
+++ b/Doc/library/difflib.rst
@@ -421,7 +421,7 @@
       is not changed.
 
 
-   .. method:: find_longest_match(alo, ahi, blo, bhi)
+   .. method:: find_longest_match(alo=0, ahi=None, blo=0, bhi=None)
 
       Find longest matching block in ``a[alo:ahi]`` and ``b[blo:bhi]``.
 
@@ -458,6 +458,9 @@
 
       This method returns a :term:`named tuple` ``Match(a, b, size)``.
 
+      .. versionchanged:: 3.9
+         Added default arguments.
+
 
    .. method:: get_matching_blocks()