Extra unprintable chars in code blocks

[damithc]

When I copy-paste python code from this section, intellij indicates there are some unprintable chars.
https://nus-te3201.github.io/2020/schedule/week10/topics-py.html#appendix-d-unit-testing

I wonder if they have been added by MarkBind. The code in the source file doesn't seem to have the same chars. https://github.com/nus-te3201/2020/blob/te3201/programming/unittesting/text.md

[openorclose]

It was caused by the line numbers pr #991, where an unprintable character was added to empty lines.

I'll submit a pr in the morning.