Convert keyword elements correctly

[MattiasRosell]

I noticed that the <del> tag was not converted into htpy's del_ correctly. The formatters also can not handle importing python's del keyword and would crash. Ruff would not output anything while Black would output the unformatted code.

I fixed these issues by:

• Replacing div with div_ when converting html using html2htpy.
• Adding a test checking that div is successfully converted to div_.
• Adding consistent behaviour between Ruff and Black when the input can not be formatted.

@pelme then found out that htpy did not convert the del_ element to html correctly. So we fixed that as well.

• Fix the del_ element not converting into <del> correctly.
• Only remove underscore from elements named after Python's keywords.

[pelme]

praise: very nice, fixing annoyances in html2htpy is really helpful and makes it nicer to use!

[MattiasRosell]

Updated branch to fix conflicts

[pelme]

praise: very nice, having a html2htpy that works reliable is nice!