Fix test setup, update dependencies

[greschd]

Fix test setup on Linux by introducing a small sleep after launching the server. The reason why this is needed is not completely clear, I suspect a problem with binding to the port. In PyACP tests (which launch via docker compose), we've never seen this problem.

Improve the error handling in the helper context managers of the tests.

Update the poetry lockfile.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.77%. Comparing base (cbbc691) to head (ed04ac8).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #137   +/-   ##
=======================================
  Coverage   89.77%   89.77%           
=======================================
  Files           3        3           
  Lines          88       88           
=======================================
  Hits           79       79           
  Misses          9        9           

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