capture_output is python 3.7 onwards

[manfredsc]

Not sure what's the state of python version support in this project, at least
it seems pyproject.toml is a bit outdated.

xlsx2csv 0.8.2 does not work with python 3.6, as capture_output is python 3.7 onwards.

If backwards compatibility is desired, I would suggest to replace in test/run
capture_output = True with stdout = subprocess.PIPE, stderr = subprocess.PIPE
which should be equivalent.