python 3.8 replace async with async_

[time4e]

Just an fyi I've been running retweet-bot for sometime now under FreeBSD 12 using 3.5, 3.5 is now longer available in FreeBSD 13-release and when running python retweet-bot under 3.8 the following error occurred

  File "/usr/local/lib/python3.8/site-packages/tweepy/streaming.py", line 357
    if async:
       ^
SyntaxError: invalid syntax

This fix for this was rather simple and that was just to replace all lines containing async with async_ in the referenced streaming.py after that retweeting now works as expected.