Switch to async/await and apply Flake8 to all files

[aiudirog]

Here is the PR for #144 which converts all the generator based coroutines to async/await syntax.

In addition, it:

• adds PyPy3 to the testing environments
• updates timeout test cases to use socket/connect_timeout to avoid the deprecation warning for the old timeout argument
• fixes the SSL timeout test on newer PyPy2 versions
• fixes the tox environments to ensure Flake8 runs on all files, not just the tests
• updates all necessary files to pass Flake8

[codecov[bot]]

Codecov Report

Base: 84.31% // Head: 83.26% // Decreases project coverage by -1.05% :warning:

Coverage data is based on head (7f82b6f) compared to base (220aabf). Patch coverage: 68.14% of modified lines in pull request are covered.

:exclamation: Current head 7f82b6f differs from pull request most recent head e7d5dba. Consider uploading reports for the commit e7d5dba to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #159      +/-   ##
==========================================
- Coverage   84.31%   83.26%   -1.06%     
==========================================
  Files          44       43       -1     
  Lines        4164     3920     -244     
==========================================
- Hits         3511     3264     -247     
- Misses        653      656       +3     

Impacted Files	Coverage Δ
thriftpy2/protocol/json.py	82.75% <ø> (-13.99%)	:arrow_down:
thriftpy2/transport/_ssl.py	31.74% <ø> (+0.49%)	:arrow_up:
thriftpy2/contrib/aio/protocol/binary.py	56.80% <47.54%> (+1.06%)	:arrow_up:
thriftpy2/rpc.py	79.16% <50.00%> (-2.78%)	:arrow_down:
thriftpy2/utils.py	79.16% <50.00%> (ø)
thriftpy2/contrib/aio/protocol/compact.py	60.63% <56.71%> (+0.46%)	:arrow_up:
thriftpy2/contrib/aio/rpc.py	83.72% <75.00%> (-0.73%)	:arrow_down:
thriftpy2/contrib/aio/processor.py	84.00% <81.25%> (-0.32%)	:arrow_down:
thriftpy2/contrib/aio/client.py	82.45% <84.61%> (ø)
thriftpy2/contrib/aio/socket.py	78.14% <87.50%> (+0.12%)	:arrow_up:
... and 23 more

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[rlratzel]

Hi, assuming the merge conflicts can be resolved, will this PR be approved and merged any time soon?

[aiudirog]

@ethe I can get this back in shape if you're interested in approving and merging it

[ethe]

Hi @aiudirog it looks good to me, I think if conflicts are all resolved we can merge it immediately