Revert invalid use of io.StringIO in utils/profiler.py

[luke-zhu]

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My previous Python 3 compatibility PR converted cStringIO.StringIO to io.StringIO. This doesn't work in Python 2 because io.StringIO operates only unicode input.

[lukecwik]

@charlesccychen do you mind taking a first pass and then reaching out to a contributor for review?

[aaltay]

LGTM. I see some python test errors, but they are related to recently fixed issues.