Use utf8 encoding

In this MR, we made some changes to use Utf8 encoding on prepended python paths and frame filename. Utf8 is the encoding used for Strings in Python3.

Here we could call toLocal8bit() to get and use the local encoding of the system, as it's done in paraview, and everything will work under UNIX as Utf8 is the local encoding, most of the time. But on Windows it is not always the case so we can encounter decoding issues with the python wrapping functions. So it is better to use toUtf8() in our case.

Note : In PV5.9, all toLocal8bit() have been recently changed to toUtf8().

@arnaud.billon PTAL