fix: file open mode fixed to w

[ketsapiwiq]

conllXtostandoff.py was using an incorrect mode to create output files.
It resulted in:

python3 tools/conllXtostandoff.py -o test data/UD_Turkish-IMST/tr_imst-ud-test.conllu
Writing output to test
Error processing data/UD_Turkish-IMST/tr_imst-ud-test.conllu: b"can't have text and binary mode at once"

##############################################################################
#
# WARNING: error in processing 1/1 files, output is incomplete!
#
##############################################################################