[Stdlib] Add PythonObject.__contains__

Signed-off-by: rd4com <[email protected]>

docs/manual/python/types.ipynb

@@ -193,17 +193,13 @@
                 "    var py_set = Python.evaluate('set([2, 3, 5, 7, 11])')\n",
                 "    var num_items = len(py_set)\n",
                 "    print(num_items, \" items in set.\")  # prints \"5 items in set\"\n",
-                "    print(py_set.__contains__(6))       # prints \"False\""
+                "    print(6 in py_set)       # prints \"False\""
             ]
         },
         {
             "cell_type": "markdown",
             "metadata": {},
             "source": [
-                "TODO: You should be able to use the expression `6 in py_set`. However, because\n",
-                "of the way `PythonObject` currently works, you need to call the \n",
-                "`__contains__()` method directly.\n",
-                "\n",
                 "Some Mojo APIs handle `PythonObject` just fine, but sometimes you'll need to \n",
                 "explicitly convert a Python value into a native Mojo value. \n",
                 "\n",