Fix #7964: autodoc: Tuple in default value is wrongly rendered

This implements tuple literal support to sphinx.pycode.ast.unparse().

tests/test_pycode_ast.py

@@ -53,7 +53,7 @@ from sphinx.pycode import ast
     ("+ a", "+ a"),                             # UAdd
     ("- 1", "- 1"),                             # UnaryOp
     ("- a", "- a"),                             # USub
-    ("(1, 2, 3)", "1, 2, 3"),                   # Tuple
+    ("(1, 2, 3)", "(1, 2, 3)"),                   # Tuple
     ("()", "()"),                               # Tuple (empty)
 ])
 def test_unparse(source, expected):