Bad performance in _unescape_special_chars

[max-wittig]

Describe the bug

python-markdown2/lib/markdown2.py

Line 2225 in adf4e81

def _unescape_special_chars(self, text: str) -> str:

To Reproduce
Pretty hard as its a big markdown file

Expected behavior
A clear and concise description of what you expected to happen.

Debug info
Version of library being used:

Any extras being used:

Additional context
Add any other context about the problem here.

[nicholasserra]

Thanks for the report!

[Crozzers]

Are you able to provide any steps for reproducing the issue? Maybe a script that generates the markdown snippet that causes the problem? Or perhaps a link to a sample file?