Solution webview doesn't highlight python3 language

[Vigilans]

🐛 Bug Report

The highlight.js seems unable to recognize python3, and will not highlight the code fences.

To Reproduce

Execute Show Top Voted Solution on any problem with language python3, then we'll get

Expected behavior

Alter Solution.lang: python3 to python when passed to highlight.js.

[jdneo]

@Vigilans I think this should be fixed in #224, right?

[Vigilans]

@jdneo Yes, I have tested it.