stdout does not have flush

[alexforencich]

When working on the long log line fix, I encountered an exception from this line:

cocotb-test/cocotb_test/simulator.py

Line 371 in 4a5f1d2

self.process.stdout.flush()

It seems that stdout is a StreamReader object, which does not have flush(). Not sure if this line should be removed, or if there is an alternative method for doing something equivalent.