bpo-32912: Replace a DeprecationWarning with a SyntaxWarning (GH-9652)

for invalid escape sequences in string and bytes literals.
diff --git a/Doc/reference/lexical_analysis.rst b/Doc/reference/lexical_analysis.rst
index 84e8c78..e1d88ef 100644
--- a/Doc/reference/lexical_analysis.rst
+++ b/Doc/reference/lexical_analysis.rst
@@ -559,8 +559,11 @@
 unrecognized escapes for bytes literals.
 
    .. versionchanged:: 3.6
-      Unrecognized escape sequences produce a DeprecationWarning.  In
-      some future version of Python they will be a SyntaxError.
+      Unrecognized escape sequences produce a :exc:`DeprecationWarning`.
+
+   .. versionchanged:: 3.8
+      Unrecognized escape sequences produce a :exc:`SyntaxWarning`.  In
+      some future version of Python they will be a :exc:`SyntaxError`.
 
 Even in a raw literal, quotes can be escaped with a backslash, but the
 backslash remains in the result; for example, ``r"\""`` is a valid string