Partially implement flush for RFC2217

[puddly]

RFC2217 doesn't have a way to "flush". We instead "flush" the TCP socket by sending a no-op request and receiving a response.

test_fast_open_close as a test relies on behavior that not all backends technically have: enqueuing data to send and then closing the port only works on some backends.

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.73%. Comparing base (e062529) to head (ee9ef58).
⚠️ Report is 2 commits behind head on dev.

Files with missing lines	Patch %	Lines
serialx/platforms/serial_rfc2217/__init__.py	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #75      +/-   ##
==========================================
- Coverage   90.77%   90.73%   -0.04%     
==========================================
  Files          21       22       +1     
  Lines        3349     3367      +18     
==========================================
+ Hits         3040     3055      +15     
- Misses        309      312       +3     

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