`async` is a reserved keyword in Python 3.7

[michael-k]

opbeat_python/opbeat/base.py

Line 122 in 02af225

async=None, async_mode=None, traces_send_freq_secs=None,

[beniwohli]

Thanks @michael-k :) we already started phasing out the async keyword argument in favour of async_mode a while ago, and will remove it well before Python 3.7 is released (probably in time for beta1 of Python 3.7).