Drop obsolete html5lib package

[orlitzky]

I'm pretty sure this package is obsolete, let's see.

[jhpalmieri]

This looks okay to me. Sage builds and passes tests, and html5lib is not mentioned anywhere in the source code except in its own build directory.

[kwankyu]

I don't know if this is worth to be kept or not. But this is a standard package. What is the drop procedure of a standard package? I couldn't find anything in the developer manual.

It seems reasonable to me there is a one-year deprecation period when we drop a (standard) package. Or needs a discussion in sage-devel?

Is there an established procedure? There must be precedents. @mkoeppe @jhpalmieri ?

[jhpalmieri]

I find it very unlikely that someone both (a) relies on html5lib in Sage and (b) can't pip-install it themselves. I don't see a need for a deprecation period. About two years ago we removed ATLAS (#30350) and mpir (#32549) without any deprecation, so there is precedent.

[kwankyu]

OK. Thanks.

[mkoeppe]

I agree with John that a deprecation period is not needed for changes like this. This package was added because it was a dependency of another standard package, bleach, which is a dependency of the user-facing package nbconvert, which is invoked by a Jupyter menu item.

By the way, html5lib is not really "obsolete". We just don't need to install it any more because bleach started to vendor its own copy of html5lib 1.1 at some point.

[vbraun]

merge conflict

[github-actions]

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