Improper un-escape of underscore _x005F_
Line # 19 of reader/string.py has improper un-escape of underscore "x005F", which accidently removes "x005F_" substring from the original content.
For example, "x005F_" and "x005F_x005F" are entered in Excel, the escaped strings, "x005F_" and "x005F_x005F_x005F_x005F" are stored in the sharedStrings.xml The unescaped results are expected to be the original strings. However, the readouts using openpyxl are "" and "_" due to the incorrect process at line # 19 in reader/string.py.
class CellRichText.from_tree might have the same issue.
Edited by Shu-Hsien Chu