Support parsing of async generators in non-async functions (#165)

This is a new syntax added in python3.7, so black can't verify that reformatting will not change the ast unless black itself is run with 3.7. We'll need to change the error message black gives in this case. @ambv any ideas?

Fixes #125.

blib2to3/pgen2/tokenize.py

@@ -516,13 +516,14 @@ def generate_tokens(readline):
                         stashed = tok
                         continue
 
-                    if token == 'def':
+                    if token in ('def', 'for'):
                         if (stashed
                                 and stashed[0] == NAME
                                 and stashed[1] == 'async'):
 
-                            async_def = True
+                            if token == 'def':
-                            async_def_indent = indents[-1]
+                                async_def = True
+                                async_def_indent = indents[-1]
 
                             yield (ASYNC, stashed[1],
                                    stashed[2], stashed[3],

tests/data/python37.py

@@ -0,0 +1,13 @@
+#!/usr/bin/env python3.7
+
+def f():
+    return (i*2 async for i in arange(42))
+
+# output
+
+
+#!/usr/bin/env python3.7
+
+
+def f():
+    return (i * 2 async for i in arange(42))

tests/test_black.py

@@ -411,6 +411,16 @@ def test_stub(self) -> None:
         self.assertFormatEqual(expected, actual)
         black.assert_stable(source, actual, line_length=ll, mode=mode)
 
+    @patch("black.dump_to_file", dump_to_stderr)
+    def test_python37(self) -> None:
+        source, expected = read_data("python37")
+        actual = fs(source)
+        self.assertFormatEqual(expected, actual)
+        major, minor = sys.version_info[:2]
+        if major > 3 or (major == 3 and minor >= 7):
+            black.assert_equivalent(source, actual)
+        black.assert_stable(source, actual, line_length=ll)
+
     @patch("black.dump_to_file", dump_to_stderr)
     def test_fmtonoff(self) -> None:
         source, expected = read_data("fmtonoff")