Fix Python 3 issues when generating preview snippets

Fix Python 3 issues when generating preview snippets

The log file generated by latex can contain strings encoded in
whatever supported encoding. Instead of guessing the encoding,
it is better to open it in binary mode and then performing the
necessary comparisons as "bytes". In order to do this, the
strings are encoded in utf8, so that, for example, b"pythön" is
encoded as "pyth\xc3\xb6n" (7 bytes). Of course, this means that
we can only successfully perform comparisons with ascii strings.
However, this is what we actually do, as we only search for
ascii strings in the log file.