revng daemon: identify valid port

This commit resolves a non-deterministic test failure in `revng daemon`.

When running daemon tests, `randint` was used for selecting the listen
port, this caused test failures when the same port number was used for
reused. This has been replaced with the use of port 0 that guarantees
the use of an unused port and psutil to find the port number once it has
been bound by the daemon process.

python/requirements.txt

@@ -23,3 +23,4 @@ pytest
 requests
 requests-toolbelt
 gql
+psutil