Celery を Python3.7 で使うとき、 backend に redis を指定すると syntax error がでて、動かない。

以下のような syntax error が起きてしまい、 backend に redis が指定できない。

```
[2019-02-07 19:50:49,175: CRITICAL/MainProcess] Unrecoverable error: SyntaxError('invalid syntax', ('$HOME/pyvenv/py37/lib/python3.7/site-packages/celery/backends/redis.py', 22, 19, 'from . import async, base\n'))
Traceback (most recent call last):
  File "$HOME/pyvenv/py37/lib/python3.7/site-packages/kombu/utils/objects.py", line 42, in __get__
    return obj.__dict__[self.__name__]
KeyError: 'backend'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "$HOME/pyvenv/py37/lib/python3.7/site-packages/celery/worker/worker.py", line 205, in start
    self.blueprint.start(self)
  File "$HOME/pyvenv/py37/lib/python3.7/site-packages/celery/bootsteps.py", line 115, in start
    self.on_start()
  File "$HOME/pyvenv/py37/lib/python3.7/site-packages/celery/apps/worker.py", line 139, in on_start
    self.emit_banner()
  File "$HOME/pyvenv/py37/lib/python3.7/site-packages/celery/apps/worker.py", line 154, in emit_banner
    ' \n', self.startup_info(artlines=not use_image))),
  File "$HOME/pyvenv/py37/lib/python3.7/site-packages/celery/apps/worker.py", line 217, in startup_info
    results=self.app.backend.as_uri(),
  File "$HOME/pyvenv/py37/lib/python3.7/site-packages/kombu/utils/objects.py", line 44, in __get__
    value = obj.__dict__[self.__name__] = self.__get(obj)
  File "$HOME/pyvenv/py37/lib/python3.7/site-packages/celery/app/base.py", line 1196, in backend
    return self._get_backend()
  File "$HOME/pyvenv/py37/lib/python3.7/site-packages/celery/app/base.py", line 914, in _get_backend
    self.loader)
  File "$HOME/pyvenv/py37/lib/python3.7/site-packages/celery/app/backends.py", line 70, in by_url
    return by_name(backend, loader), url
  File "$HOME/pyvenv/py37/lib/python3.7/site-packages/celery/app/backends.py", line 50, in by_name
    cls = symbol_by_name(backend, aliases)
  File "$HOME/pyvenv/py37/lib/python3.7/site-packages/kombu/utils/imports.py", line 56, in symbol_by_name
    module = imp(module_name, package=package, **kwargs)
  File "$HOME/.local/lib/python3.7/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
  File "<frozen importlib._bootstrap>", line 983, in _find_and_load
  File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 724, in exec_module
  File "<frozen importlib._bootstrap_external>", line 860, in get_code
  File "<frozen importlib._bootstrap_external>", line 791, in source_to_code
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "$HOME/pyvenv/py37/lib/python3.7/site-packages/celery/backends/redis.py", line 22
    from . import async, base
                      ^
SyntaxError: invalid syntax
```

Python 3.7 から async が予約語になったので、構文エラーが出たようです。

PyPi に登録しているものは、まだ update されていないので、GitHub にあるソースから update してください。(2019-02-07 現在)

`pip install --upgrade https://github.com/celery/celery/tarball/master`

issues に上がったのが、２０１８年８月７日あたりで、２０１８年８月１８日にリリースされているのですが、 修正されたのが２７日です。残念！！

個人的な予測ですが、 4.2.2 は出さずに、 4.3 になりそうな予感。

いろいろ議論がされていますが、英語がよくわかりません…。

---

参考:
- [Python 3.7 syntax error: async is a reserved keyword](https://github.com/celery/celery/issues/4849)
- [Release version 4.3 of Celery](https://github.com/celery/celery/issues/5180)