Add context to find_unused_port

[hodbn]

The helper function find_unused_port might return a port that will immediately be reused by the system.
I wrapped the helper in a contextmanager so the port is allocated to tempsock while inside the context.

This PR affects #1830, I will update it/this PR once one of them is accepted.

[codecov]

Codecov Report

Merging #1837 into master will not change coverage by %.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master     #1837   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           22        22           
  Lines         2007      2007           
=========================================
  Hits          2007      2007           

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[pquentin]

The function still says: The temporary socket is then closed and deleted, and the ephemeral port is returned.

Obviously the tests passed fine, but how does this work? Why is it not an issue to not close the socket before starting the test?

[hodbn]

The yield is just after the bind_port call and before the close(), so the block inside the with is executed while the socket is still registered and the port is reserved by the OS. I've fixed the docs :)

[pquentin]

Great, thanks!