bpo-42700: Swap descriptions in pyexpat.errors (GH-23876)

The descriptions of the `codes` and `messages` dictionaries in
`xml.parsers.expat.errors` were swapped, and this commit swaps them
back. For example, `codes` maps string descriptions of errors to numeric
error codes, not the other way around.

Doc/library/pyexpat.rst

@@ -665,10 +665,10 @@
 
 .. data:: codes
 
-   A dictionary mapping numeric error codes to their string descriptions.
+   A dictionary mapping string descriptions to their error codes.
 
    .. versionadded:: 3.2
 
 
 .. data:: messages
 
@@ -669,10 +669,10 @@
 
    .. versionadded:: 3.2
 
 
 .. data:: messages
 
-   A dictionary mapping string descriptions to their error codes.
+   A dictionary mapping numeric error codes to their string descriptions.
 
    .. versionadded:: 3.2