Cache sys.stdout instead of assuming it is equal to sys.__stdout__

When running under Mitogen, ipa_server and ipa_replica breaks execution
by overwriting sys.stdout with sys.stdout.

With Mitogen, sys.stdout != sys.stdout at this point in the code, and
changing it in this manner results in access to closed file descriptors
for future invocations. Generally, it is recommended not to use
sys.stdout and instead explicitly cache the current value of sys.stdout.