[wip] Set up yapf to do auto-formatting

[njsmith]

This is the preparatory stuff -- at the right moment we can rebase
this and then run 'yapf -rip setup.py trio' to reformat everything.

[codecov]

Codecov Report

Merging #221 into master will decrease coverage by 0.01%.
The diff coverage is 97.87%.

@@            Coverage Diff             @@
##           master     #221      +/-   ##
==========================================
- Coverage   99.15%   99.13%   -0.02%     
==========================================
  Files          67       67              
  Lines        9190     9213      +23     
  Branches      656      663       +7     
==========================================
+ Hits         9112     9133      +21     
  Misses         62       62              
- Partials       16       18       +2

Impacted Files	Coverage Δ
trio/tests/test_timeouts.py	100% <ø> (ø)	⬆️
trio/_core/tests/test_unbounded_queue.py	100% <ø> (ø)	⬆️
trio/_core/__init__.py	100% <ø> (ø)	⬆️
trio/_core/_unbounded_queue.py	100% <ø> (ø)	⬆️
trio/tests/test_abc.py	100% <ø> (ø)	⬆️
trio/_path.py	100% <ø> (ø)	⬆️
trio/_open_tcp_stream.py	96.66% <ø> (ø)	⬆️
trio/_ssl_stream_helpers.py	100% <ø> (ø)	⬆️
...tests/test_multierror_scripts/custom_excepthook.py	100% <ø> (ø)	⬆️
trio/_core/_windows_cffi.py	91.3% <ø> (ø)	⬆️
... and 53 more

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