Fix literal attribute access on Python 3.6 and 3.7

lib/astunparse/unparser.py

@@ -708,7 +708,10 @@ def _Attribute(self,t):
         # Special case: 3.__abs__() is a syntax error, so if t.value
         # is an integer literal then we need to either parenthesize
         # it or add an extra space to get 3 .__abs__().
-        if isinstance(t.value, getattr(ast, 'Constant', getattr(ast, 'Num', None))) and isinstance(t.value.n, int):
+        ast_Num = getattr(ast, 'Num', None)
+        ast_Constant = getattr(ast, 'Constant', None)
+        if ((ast_Num and isinstance(t.value, ast_Num) and isinstance(t.value.n, int))
+           or (ast_Constant and isinstance(t.value, ast_Constant) and isinstance(t.value.value, int))):
             self.write(" ")
         self.write(".")
         self.write(t.attr)

tests/common.py

@@ -215,7 +215,6 @@ def test_unary_parens(self):
         self.check_roundtrip("not True or False")
         self.check_roundtrip("True or not False")
 
-    @unittest.skipUnless(sys.version_info < (3, 6), "Only works for Python < 3.6")
     def test_integer_parens(self):
         self.check_roundtrip("3 .__abs__()")