use sys.get_asyncgen_hooks().finalizer to sniff io

[graingert]

Fixes #20 #27

[graingert]

This is based on the observation that the primary purpose of an async framework is to collect async generator objects, close them correctly and raise a resource warning.

sniffio is no longer answering the question of "what sort of object should I _async_yield", it's answering "if I de-reference a an async generator where does it go?"

[codecov]

Codecov Report

Merging #28 (9ce0ca3) into master (0cfdab8) will decrease coverage by 9.17%.
The diff coverage is 80.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
- Coverage   96.51%   87.34%   -9.17%     
==========================================
  Files           4        4              
  Lines          86      158      +72     
  Branches       17       22       +5     
==========================================
+ Hits           83      138      +55     
- Misses          2       17      +15     
- Partials        1        3       +2     

Files	Coverage Δ
sniffio/__init__.py	100.00% <ø> (ø)
sniffio/_tests/test_sniffio.py	100.00% <100.00%> (ø)
sniffio/_impl.py	68.75% <63.26%> (-22.16%)	⬇️