Traceback when using typing.Optional[Type] on xenial

[Dan Watkins]

typing in the Python 3.5.2 stdlib tracebacks when passing Type to a Union (of which Optional is a specific case). (This is typing bug #266: https://github.com/python/typing/issues/266.)

(This is particularly annoying because as a stdlib library, the fix released by upstream to PyPI can't easily be used even via pip.)

Steps to Reproduce
==================

```
lxc launch ubuntu:x reproducer
lxc exec reproducer -- python3 -c "from typing import Optional, Type; Optional[Type[BaseException]]"
```

Expected Behaviour
==================

No output, exits 0.

Actual Behaviour
================

```
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib/python3.5/typing.py", line 649, in __getitem__
    return Union[arg, type(None)]
  File "/usr/lib/python3.5/typing.py", line 552, in __getitem__
    dict(self.__dict__), parameters, _root=True)
  File "/usr/lib/python3.5/typing.py", line 512, in __new__
    for t2 in all_params - {t1} if not isinstance(t2, TypeVar)):
  File "/usr/lib/python3.5/typing.py", line 512, in <genexpr>
    for t2 in all_params - {t1} if not isinstance(t2, TypeVar)):
  File "/usr/lib/python3.5/typing.py", line 1077, in __subclasscheck__
    if super().__subclasscheck__(cls):
  File "/usr/lib/python3.5/abc.py", line 225, in __subclasscheck__
    for scls in cls.__subclasses__():
TypeError: descriptor '__subclasses__' of 'type' object needs an argument
```

[Dimitri John Ledkov (xnox)]

Yakkety and xenial are the only affected. 3.5.3 has this fix in zesty and artful.

[Brian Turek (brian-turek)]

I just ran into this yesterday and was going to submit a bug report. Given that this is a one-line fix, can we get this backported? I'm happy to make the patch file.