Upgrade pytest-asyncio to 0.26.0

[mryab]

This PR upgrades pytest_asyncio, following the migration guide given in the plugin documentation

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 85.38%. Comparing base (d20e810) to head (ad55ef1).
⚠️ Report is 26 commits behind head on master.

Files with missing lines	Patch %	Lines
hivemind/utils/asyncio.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #652      +/-   ##
==========================================
- Coverage   85.39%   85.38%   -0.01%     
==========================================
  Files          81       96      +15     
  Lines        8006     8583     +577     
==========================================
+ Hits         6837     7329     +492     
- Misses       1169     1254      +85     

Files with missing lines	Coverage Δ
hivemind/p2p/p2p_daemon.py	96.44% <100.00%> (+0.86%)	⬆️
hivemind/p2p/p2p_daemon_bindings/control.py	91.82% <100.00%> (+0.22%)	⬆️
hivemind/utils/asyncio.py	96.37% <88.88%> (-3.63%)	⬇️

... and 31 files with indirect coverage changes

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[mryab]

This is introduced to make sure we don't run into errors when trying to cancel tasks with a closed event loop. In particular, this might happen in tests once we remove the current event_loop fixture, which seems to be recommended as per the migration guide

[justheuristic]

LGTM