no-GIL/freethreaded Python support

[gi0baro]

Required steps:

• PyO3 support (Tracking issue for no-gil/freethreaded work PyO3/pyo3#4265)
• uvloop support (Add support for free-threaded (nogil) 3.13 Python MagicStack/uvloop#642)
• split workers implementation into two:
  • the existing one based on multiprocessing
  • the new one (no-gil only) based on threads
• re-implement signals handling in threaded workers (cannot be used outside main thread)
• check implications of having multiple running Python event loops (one per thread)
• find the best way to support both GIL and no-GIL Python
  • this will probably require to build different wheels for 3.13
• check internal mutability and thread-safe code is still valid in no-GIL ctx
• threaded workers means we can avoid sharing the listening socket between processes
  • should potentially fix Windows workers limit 1