HTMLParser.unescape in python 3.9

Hi.
In python 3.9.0 HTMLParser.unescape was removed so the app throws error when trying to parse:
```
$python3 --version   
             
Python 3.9.5
```
and:
```
File ".../mdv/markdownviewer.py", line 970, in formatter
    t = html_parser.unescape(t)
AttributeError: 'HTMLParser' object has no attribute 'unescape'
```