Python/Pexpect: Use upstream output encoding approach
Python/Pexpect: Use upstream output encoding approach

WinPexpect ported its Pexpect to Python 3 by using s.encode('ascii')
while upstream Pexpect did the same using s.encode("utf-8").  Use the
latter so we can merge with upstream changes cleanly.

Change-Id: Iae8da9e631b0cba0546572e7ff3ff5c8f608116a

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