Implemented trio backend

[francis-clairicia]

What's changed

• Added trio implementation of AsyncBackend interface
• Added AsyncBackend.gather() method (not abstract)
• Added AsyncBackend.getaddrinfo() and AsyncBackend.getnameinfo()
  • By default, simply call the functions in socket module within a thread.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (94f5e72) to head (8e7bfef).
Report is 52 commits behind head on main.

Additional details and impacted files

Flag	Coverage Δ
OS-Linux	99.88% <99.78%> (-0.02%)	⬇️
OS-Windows	99.78% <99.78%> (-0.01%)	⬇️
OS-macOS	99.88% <99.78%> (-0.02%)	⬇️
Py-3.11	99.89% <99.78%> (-0.01%)	⬇️
Py-3.12	99.92% <99.78%> (-0.03%)	⬇️
test-functional	82.76% <88.91%> (+0.90%)	⬆️
test-unit	89.35% <84.23%> (-0.37%)	⬇️

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