Clarify what counts as 'running'

[oremanj]

As proposed in #38, this PR updates sniffio's documentation and asyncio detection to adopt a consistent stance on what counts as "currently running". Now, user code that is invoked by an async library but is not part of an async task will still return that async library from sniffio.current_async_library().

Fixes #35. Fixes #38.

[codecov[bot]]

Codecov Report

Merging #39 (852561d) into master (0cfdab8) will increase coverage by 3.48%. The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##           master       #39      +/-   ##
===========================================
+ Coverage   96.51%   100.00%   +3.48%     
===========================================
  Files           4         4              
  Lines          86        81       -5     
  Branches       17        15       -2     
===========================================
- Hits           83        81       -2     
+ Misses          2         0       -2     
+ Partials        1         0       -1     

Impacted Files	Coverage Δ
sniffio/_impl.py	100.00% <100.00%> (+9.09%)	:arrow_up:
sniffio/_tests/test_sniffio.py	100.00% <100.00%> (ø)