add UTF-8 encoding to file open()

[mcorne]

the encoding default is not UTF-8 on all platform, eg Windows
it would be nice to add the encoding="utf-8" option to all open() calls in https://github.com/cltk/lat_models_cltk/blob/master/lemmata/collatinus/__convert.py
for example, line 28, replace:
with open("src/morphos.la") as f:
with:
with open("src/morphos.la", encoding="utf-8") as f: