Add support for sniffio

[njsmith]

See https://github.com/python-trio/sniffio

And also vxgmichel/aiostream#17

This shouldn't be merged (and tests will fail) until sniffio is
released on PyPI. But it demonstrates how to do it (and the tests pass
locally with my current sniffio checkout).

[njsmith]

Okay, sniffio is now up on pypi, so this should be ready to review/merge in a few minutes when the tests are finished...

[codecov]

Codecov Report

Merging #572 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #572      +/-   ##
==========================================
+ Coverage   99.27%   99.27%   +<.01%     
==========================================
  Files          89       89              
  Lines       10617    10651      +34     
  Branches      747      746       -1     
==========================================
+ Hits        10540    10574      +34     
  Misses         59       59              
  Partials       18       18

Impacted Files	Coverage Δ
trio/_core/tests/test_run.py	100% <100%> (ø)	⬆️
trio/_core/_run.py	100% <100%> (ø)	⬆️
trio/_sync.py	100% <0%> (ø)	⬆️
trio/_ssl.py	100% <0%> (ø)	⬆️
trio/_socket.py	100% <0%> (ø)	⬆️
trio/tests/test_socket.py	100% <0%> (ø)	⬆️
trio/tests/test_sync.py	100% <0%> (ø)	⬆️

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