PEP 483: Define AnyStr before mentioning it (#2682)

pep-0483.txt

@@ -441,9 +441,9 @@ replaced by the most-derived base class among ``t1``, etc. Examples:
 
 - Function type annotation with a constrained type variable::
 
-    S = TypeVar('S', str, bytes)
+    AnyStr = TypeVar('AnyStr', str, bytes)
 
-    def longest(first: S, second: S) -> S:
+    def longest(first: AnyStr, second: AnyStr) -> AnyStr:
         return first if len(first) >= len(second) else second
 
     result = longest('a', 'abc')  # The inferred type for result is str