fix lint and make it crash less often on typing without AsyncGenerator

Author: JelleZijlstra

mypy/checker.py

@@ -374,7 +374,11 @@ def is_async_generator_return_type(self, typ: Type) -> bool:
 
         True if `typ` is a supertype of AsyncGenerator.
         """
-        agt = self.named_generic_type('typing.AsyncGenerator', [AnyType(), AnyType()])
+        try:
+            agt = self.named_generic_type('typing.AsyncGenerator', [AnyType(), AnyType()])
+        except KeyError:
+            # we're running on a version of typing that doesn't have AsyncGenerator yet
+            return False
         return is_subtype(agt, typ)
 
     def get_generator_yield_type(self, return_type: Type, is_coroutine: bool) -> Type:

mypy/messages.py

@@ -31,7 +31,8 @@
 INVALID_RETURN_TYPE_FOR_GENERATOR = \
     'The return type of a generator function should be "Generator" or one of its supertypes'
 INVALID_RETURN_TYPE_FOR_ASYNC_GENERATOR = \
-    'The return type of an async generator function should be "AsyncGenerator" or one of its supertypes'
+    'The return type of an async generator function should be "AsyncGenerator" or one of its ' \
+    'supertypes'
 INVALID_GENERATOR_RETURN_ITEM_TYPE = \
     'The return type of a generator function must be None in its third type parameter in Python 2'
 YIELD_VALUE_EXPECTED = 'Yield value expected'