Fix Markdown unit tests on Python 3.
Review Request #11767 — Created Aug. 1, 2021 and submitted
render_markdown_from_file()was broken on Python 3, due to
some bad expectations on string types on a stream. Our unit tests for
our wrapper, introduced in release-3.0.x, were likewise using wrong
This change fixes up the unit tests to feed in byte string data instead
of Unicode text for the test.
Unit tests pass on Python 2 and 3.