Python 3.8 compatibility

Using python3.8, with -Werror

>>> from freezegun import freeze_time
>>> @freeze_time()
... async def foo():
...     pass
... 
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "[...]/.tox/py38-std-sha256/lib/python3.8/site-packages/freezegun/api.py", line 500, in __call__
    return self.decorate_coroutine(func)
  File "[...]/.tox/py38-std-sha256/lib/python3.8/site-packages/freezegun/api.py", line 689, in decorate_coroutine
    return wrap_coroutine(self, coroutine)
  File "[...]/.tox/py38-std-sha256/lib/python3.8/site-packages/freezegun/_async.py", line 9, in wrap_coroutine
    def wrapper(*args, **kwargs):
  File "/opt/python/3.8-dev/lib/python3.8/asyncio/coroutines.py", line 111, in coroutine
    warnings.warn('"@coroutine" decorator is deprecated since Python 3.8, use "async def" instead',
DeprecationWarning: "@coroutine" decorator is deprecated since Python 3.8, use "async def" instead

According to the release notes: https://docs.python.org/3.8/whatsnew/changelog.html#changelog

bpo-36921: Deprecate @coroutine for sake of async def.

[tirkarthi]

One note is that the freezegun supports 3.4 which reached EoL. async def syntax is supported only from 3.5 and above

[FedericoCeratto]

Related to https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=944026