Add compatibility with aiohttp 2.3.1

[ppaeps]

The finish_connections method has been deprecated (ref: aio-libs/aiohttp#2006) and was replaced
with shutdown. No functional change.

============================= test session starts ==============================
platform freebsd11 -- Python 3.6.3, pytest-3.2.3, py-1.4.34, pluggy-0.4.0
Using event loop: asyncio
rootdir: /wrkdirs/usr/ports/devel/py3-threema-msgapi/work/threema-msgapi-sdk-python-3.0.6, inifile:
plugins: asyncio-0.8.0
collected 104 items

tests/test_api.py ...................................................................
tests/test_base.py ...
tests/test_blocking_api.py ......
tests/test_callback.py ......
tests/test_cli.py ......................

========================= 104 passed in 23.54 seconds ==========================

[ppaeps]

I'm not sure why Travis is complaining about this. I just checked on Python 3.5 and 3.6 with asyncio and it's all happy here. Perhaps it's just Python on Linux that's broken? I do not have Linux systems to test on.

[lgrahl]

I'm not sure why Travis is complaining about this.

Flaky tests. 😒 I've restarted them.

[codecov-io]

Codecov Report

Merging #38 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #38   +/-   ##
=======================================
  Coverage   60.22%   60.22%           
=======================================
  Files           9        9           
  Lines        1335     1335           
  Branches      129      129           
=======================================
  Hits          804      804           
  Misses        484      484           
  Partials       47       47

Impacted Files	Coverage Δ
threema/gateway/e2e.py	78.87% <100%> (ø)	⬆️

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[lgrahl]

I assume there's no rush for release, correct?

[ppaeps]

Correct. No rush!

Thanks for the merge.