Add more async lifecycle tests and fix potential POSIX cancellation fd leak

[puddly]

This PR adds an explicit test suite for the asyncio.Protocol state machine pattern and aligns platform behavior around poorly-timed cancellation. These aren't things normally hit at runtime often but it's not good practice to leak file descriptors, especially when we use flock.

We also plug a few more cancellation point leaks. This pattern is a bit annoying to deal with because any await point can potentially be cancelled but with the asterisk that we are often awaiting on the result of a thread pool executor, which cannot be cancelled. This requires a bit of coordination with future callbacks and such.

For Windows, we now intentionally detach the file handle from PyHANDLE and let it "leak" for consistency with POSIX, since the cleanup code should work without a magical __del__.

[codecov]

Codecov Report

❌ Patch coverage is 93.84615% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.21%. Comparing base (b068974) to head (96c29c3).

Files with missing lines	Patch %	Lines
serialx/platforms/serial_rfc2217/__init__.py	81.81%	2 Missing ⚠️
serialx/platforms/serial_win32.py	88.23%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #93      +/-   ##
==========================================
+ Coverage   91.46%   92.21%   +0.74%     
==========================================
  Files          22       22              
  Lines        3599     3621      +22     
==========================================
+ Hits         3292     3339      +47     
+ Misses        307      282      -25     

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