Python 3.7 support

[ppawiggers]

opencensus-python does not support Python 3.7, for example it's using a forbidden name (since Python 3.7):

Traceback (most recent call last):
  File "main.py", line 4, in <module>
    from opencensus.stats.exporters import stackdriver_exporter as stackdriver
  File "/Users/peter/.virtualenvs/octest/lib/python3.7/site-packages/opencensus/stats/exporters/stackdriver_exporter.py", line 23
    from opencensus.common.transports import async
                                                 ^
SyntaxError: invalid syntax

[zoidyzoidzoid]

@ppawiggers Can you do from opencensus.common.transports.async import AsyncTransport?

[zoidyzoidzoid]

PRs are always welcome too.

We could potentially do something like rename it to a better name and still export it as async on Python 3.6 and earlier, and just use a better name going forward.

[c24t]

Fixed in #362.