version 2.2.3
release v2.2.3: remove unused NseConnection module

nsett.py had no callers anywhere in pysen itself (no project.scripts
entry, no imports outside its own two ad-hoc misc/ driver scripts, no
test/ coverage) -- its only real consumer is the separate
bl15-nse-control package's NseControl. Removed the module along with
misc/test_nsett.py, misc/test_pythia.py, and the now-moot
doc/nsett-socket-review.md, after first fixing the two socket bugs it
documented (missing TCP line reassembly, busy-loop on EOF).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>