Low-level API ( AsyncBackend ): Added create_fair_lock() method

[francis-clairicia]

What's changed

• Added AsyncBackend.create_fair_lock()
  • There is a library-agnostic default implementation that uses Event objects.
• The fair Locks are used when sending data over a socket:
  • Clients:
    • AsyncTCPNetworkClient.send_packet()
    • AsyncUDPNetworkClient.send_packet()
  • Servers:
    • AsyncStreamClient.send_packet()

Miscellaneous

• AsyncDatagramServer (trio backend): Fixed potential crash when calling concurrently send_to and socket.sendto() would block.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (ff8b8a7) to head (83c0a74).
Report is 51 commits behind head on main.

Additional details and impacted files

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OS-Linux	99.88% <100.00%> (+<0.01%)	⬆️
OS-Windows	99.76% <100.00%> (+<0.01%)	⬆️
OS-macOS	99.88% <100.00%> (+<0.01%)	⬆️
Py-3.11	99.89% <100.00%> (+<0.01%)	⬆️
Py-3.12	99.92% <100.00%> (+<0.01%)	⬆️
test-functional	82.90% <97.89%> (+0.14%)	⬆️
test-unit	89.21% <56.84%> (-0.31%)	⬇️

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