md2html: use utf-8 and flush output buffer

Otherwise we get the classic Python UTF-8 errors, and the text is all
out of order. While we're at it, switch to python3 so we only have to
support one set of oddball semantics.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Suggested-by: Daniel Campbell <dlcampbell@gmx.com>