Check a variable against Union type at runtime in Python 3.6

In Python 3.8 and later, the approach suggested by MSeifert and Richard Xia can be improved by not using the undocumented attributes __origin__ and __args__. This functionality is provided by the new functions typing.get_args(tp) and typing.get_origin(tp):

>> from typing import Union, get_origin, get_args
>> x = Union[int, str]
>> get_origin(x), get_args(x)
(typing.Union, (<class 'int'>, <class 'str'>))
>> get_origin(x) is Union
True
>> isinstance(3, get_args(x))
True
>> isinstance('a', get_args(x))
True
>> isinstance([], get_args(x))
False

P.S.: I know that the question is about Python 3.6 (probably because this was the newest version at the time), but I arrived here when I searched for a solution as a Python 3.8 user. I guess that others might be in the same situation, so I thought that adding a new answer here makes sense.