Fix #6347: autodoc: crashes with a plain Tuple on Python 3.6 and 3.5

tests/test_util_inspect.py

@@ -196,7 +196,7 @@ def test_Signature_partialmethod():
 
 def test_Signature_annotations():
     from typing_test_data import (f0, f1, f2, f3, f4, f5, f6, f7, f8, f9, f10,
-                                  f11, f12, f13, f14, f15, f16, f17, Node)
+                                  f11, f12, f13, f14, f15, f16, f17, f18, Node)
 
     # Class annotations
     sig = inspect.Signature(f0).format_args()
@@ -273,6 +273,9 @@ def test_Signature_annotations():
     sig = inspect.Signature(f17).format_args()
     assert sig == '(*, arg3, arg4)'
 
+    sig = inspect.Signature(f18).format_args()
+    assert sig == '(self, arg1: Union[int, Tuple] = 10) -> List[Dict]'
+
     # type hints by string
     sig = inspect.Signature(Node.children).format_args()
     if (3, 5, 0) <= sys.version_info < (3, 5, 3):