Fix support for python 3.9

[danielnelson]

I have been using taskgroup for Python versions 3.9 and 3.10, it works quite well. I have run into one issue with the latest 0.0.0a4 release on PyPI, I get this error in some situations during cancel:

Traceback (most recent call last):
  ... snip ...
  File "/usr/lib/python3.9/asyncio/tasks.py", line 413, in wait
    return await _wait(fs, timeout, return_when, loop)
  File "/usr/lib/python3.9/asyncio/tasks.py", line 525, in _wait
    await waiter
asyncio.exceptions.CancelledError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3.9/unittest/async_case.py", line 64, in _callTestMethod
    self._callMaybeAsync(method)
  File "/usr/lib/python3.9/unittest/async_case.py", line 87, in _callMaybeAsync
    return self._asyncioTestLoop.run_until_complete(fut)
  File "/usr/lib/python3.9/asyncio/base_events.py", line 647, in run_until_complete
    return future.result()
  File "/usr/lib/python3.9/unittest/async_case.py", line 101, in _asyncioLoopRunner
    ret = await awaitable
  ... snip ...
  File "/home/dbn/usr/py-3.9/lib/python3.9/site-packages/taskgroup/taskgroups.py", line 71, in __aexit__
    if self._parent_cancel_requested and not self._parent_task.uncancel():
AttributeError: '_asyncio.Task' object has no attribute 'uncancel'

I noticed that HEAD has this fixed, to get it to work with Python 3.9 I just needed to switch some type annotations back to the Python 3.9 style, using Union and Optional.

[Gobot1234]

The better IMO way to fix this is to use __future__.annotations

[danielnelson]

That's a good idea, I have updated the PR to use this method.