get_type_hints(): find the right globalns for classes

This makes the default behavior (without specifying `globalns` manually) more
predictable for users.

Implementation assumes the class has a `__module__` attribute and that module
is present in `sys.modules`.

This is backwards compatible, will just raise fewer exceptions in naive user
code.

Author: Lukasz Langa

src/test_typing.py

@@ -1660,7 +1660,7 @@ def test_get_type_hints_modules(self):
 
     @skipUnless(PY36, 'Python 3.6 required')
     def test_get_type_hints_classes(self):
-        self.assertEqual(gth(ann_module.C, ann_module.__dict__),
+        self.assertEqual(gth(ann_module.C),  # gth will find the right globalns
                          {'y': Optional[ann_module.C]})
         self.assertIsInstance(gth(ann_module.j_class), dict)
         self.assertEqual(gth(ann_module.M), {'123': 123, 'o': type})

src/typing.py

@@ -1494,7 +1494,10 @@ def get_type_hints(obj, globalns=None, localns=None):
     if getattr(obj, '__no_type_check__', None):
         return {}
     if globalns is None:
-        globalns = getattr(obj, '__globals__', {})
+        if isinstance(obj, type):
+            globalns = vars(sys.modules[obj.__module__])
+        else:
+            globalns = getattr(obj, '__globals__', {})
         if localns is None:
             localns = globalns
     elif localns is None: