Add missing sock.close() to rpc _connect()

[ACharlyR]

The socket was not closed if the _connect() function timed out. This lead to an unclosed socket every time a second instance of pyvisa attempted to connect to a device while the first instance of pyvisa still had an open connection.

With this fix, the second instance can back off and try again at a later time without reaching an irrecoverable state.

• [ ] Closes # (insert issue number if relevant)
• [x] Executed black . && isort -c . && flake8 with no errors
• [ ] The change is fully covered by automated unit tests
• [ ] Documented in docs/ as appropriate
• [x] Added an entry to the CHANGES file

[codecov-commenter]

Codecov Report

Merging #388 (1d767e0) into main (c33f4f2) will decrease coverage by 0.01%. The diff coverage is 0.00%.

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@@            Coverage Diff             @@
##             main     #388      +/-   ##
==========================================
- Coverage   23.05%   23.04%   -0.01%     
==========================================
  Files          22       22              
  Lines        3292     3293       +1     
  Branches      456      456              
==========================================
  Hits          759      759              
  Misses       2516     2516              
- Partials       17       18       +1     

Flag	Coverage Δ
unittests	23.04% <0.00%> (-0.01%)	:arrow_down:

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Files Changed	Coverage Δ
pyvisa_py/protocols/rpc.py	21.99% <0.00%> (-0.04%)	:arrow_down: