Change asyncio detection to use get_running_loop

[tapetersen]

Attempt to solve #35

Not sure if the other tests are still needed (get_running_loop appeared in 3.7).
Added a test to ensure this works and could add more if needed.
Havn't tested under different event loops and for now only ran the tests on 3.10

[codecov]

Codecov Report

Merging #36 (ec7d250) into master (0cfdab8) will decrease coverage by 5.85%.
The diff coverage is 65.38%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #36      +/-   ##
==========================================
- Coverage   96.51%   90.65%   -5.86%     
==========================================
  Files           4        4              
  Lines          86      107      +21     
  Branches       17       16       -1     
==========================================
+ Hits           83       97      +14     
- Misses          2        9       +7     
  Partials        1        1              

Impacted Files	Coverage Δ
sniffio/_impl.py	74.35% <18.18%> (-16.56%)	⬇️
sniffio/_tests/test_sniffio.py	100.00% <100.00%> (ø)

[tapetersen]

Not sure about backwards compatibility guarantees. I think the old part with Task could be removed if we don't need to support <3.7 . The windows errors in CI I don't really understand but it feels like they aren't related to my code.

[oremanj]

Thanks for the PR! Closing in favor of #39 which is a bit more streamlined, but we're planning to adopt these semantics.