Migrate sync IO backends to nonblocking descriptors/handles

[puddly]

Currently, the sync serial interface relies on blocking reads/writes. While simple, this has the downside of not fully respecting read/write timeouts and having poor behavior when it comes to reading partial data with a timeout. This slight re-implementation switches all file descriptors/device handles to nonblocking mode and relies on kernel events to wake on availability/data (and a guaranteed read/write), as opposed to relying on the read/write call itself erroring appropriately.

Fixes #102 as a side effect

[codecov]

Codecov Report

❌ Patch coverage is 90.78947% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.05%. Comparing base (779a495) to head (62a0b50).

Files with missing lines	Patch %	Lines
serialx/platforms/serial_posix.py	79.31%	6 Missing ⚠️
serialx/platforms/serial_rfc2217/__init__.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #122      +/-   ##
==========================================
- Coverage   94.08%   94.05%   -0.04%     
==========================================
  Files          24       24              
  Lines        4025     4055      +30     
==========================================
+ Hits         3787     3814      +27     
- Misses        238      241       +3     

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