Fix bug in async function scope checking (#511)

* Fix bug in async function scope checking

Previously attempting to check an async function scoped tree would
immediately error because tuples are not callable.

The test added here is a direct copy of 'test_scope_function' as
I'm not sure what (if anything) should specifically be tested for
async functions, though it serves to prove the fix.

* Skip async function test where it doesn't apply

* Clarify that this is a smoke test for async function segments

Author: PeterJCLaw

pyflakes/checker.py

@@ -793,7 +793,7 @@ class Checker(object):
         ast.DictComp: GeneratorScope,
     }
     if PY35_PLUS:
-        _ast_node_scope[ast.AsyncFunctionDef] = FunctionScope,
+        _ast_node_scope[ast.AsyncFunctionDef] = FunctionScope
 
     nodeDepth = 0
     offset = None

pyflakes/test/test_code_segment.py

@@ -1,7 +1,9 @@
+from sys import version_info
+
 from pyflakes import messages as m
 from pyflakes.checker import (FunctionScope, ClassScope, ModuleScope,
                               Argument, FunctionDefinition, Assignment)
-from pyflakes.test.harness import TestCase
+from pyflakes.test.harness import TestCase, skipIf
 
 
 class TestCodeSegments(TestCase):
@@ -124,3 +126,7 @@ def bar(f, g=1, *h, **i):
         self.assertIsInstance(function_scope_bar['g'], Argument)
         self.assertIsInstance(function_scope_bar['h'], Argument)
         self.assertIsInstance(function_scope_bar['i'], Argument)
+
+    @skipIf(version_info < (3, 5), 'new in Python 3.5')
+    def test_scope_async_function(self):
+        self.flakes('async def foo(): pass', is_segment=True)