patch 8.2.1600: Vim9: cannot use "true" with deepcopy()

Problem:    Vim9: cannot use "true" with deepcopy().
Solution:   Use tv_get_bool_chk(). (closes #6867)

src/testdir/test_vim9_func.vim

@@ -1272,6 +1272,8 @@ def Test_copy_return_type()
     res += n
   endfor
   assert_equal(6, res)
+
+  dl = deepcopy([1, 2, 3], true)
 enddef
 
 def Test_extend_return_type()