Correct the description of the 3.7 change in urllib.parse.quote (GH-17065)

`~` is now treated as an unreserved character (i.e. it doesn't get quoted), not a reserved one.
(cherry picked from commit f49f6ba)

Co-authored-by: Роман Донченко <[email protected]>

Author: miss-islington

Doc/library/urllib.parse.rst

@@ -520,7 +520,7 @@ task isn't already covered by the URL parsing functions above.
 
    .. versionchanged:: 3.7
       Moved from :rfc:`2396` to :rfc:`3986` for quoting URL strings. "~" is now
-      included in the set of reserved characters.
+      included in the set of unreserved characters.
 
    The optional *encoding* and *errors* parameters specify how to deal with
    non-ASCII characters, as accepted by the :meth:`str.encode` method.