Fix#778 - HTMLParser.unescape was removed from Python since version 3.9.0a1

[Kirija]

…ause deprecated

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Proposed changes

HTMLParser.unescape was deprecated and removed from Python since version 3.9.0a1
So i just use html.unescape() instead of the original HTMLParser.unescape()

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[coveralls]

Coverage decreased (-0.2%) to 73.249% when pulling ecfc821 on Kirija:master into f9a9a26 on coursera-dl:master.

[heino]

Duplicate of #789,

which also catches the issue futrher down the file, and preserves backward compatiblity.

[erensunerr]

Hey! Please merge this fix. I had to fix it by hand while using it today 😭.