Fixes #13051 - Allow table cells to be aligned.

[kkthxbye-code]

Fixes: #13051

See reasoning for CSS changes here: #13051 (comment)

[jeremystretch]

@kkthxbye-code in what I can only assume is a pure coincidence and not a personal slight against you, Markdown 1.5 was just released yesterday and removes the blocker on Markdown 3.4. Should we move forward with upgrading Markdown now instead?

[kkthxbye-code]

Tried it out briefly. It installs fine, haven't checked for differences in mkdocs though, but it builds. However I wanted the upgrade because the TableExtension would use the style attribute instead, which makes the css changes unnecessary, however with bleach it's not enough to allow the style attribute:

https://bleach.readthedocs.io/en/latest/clean.html#sanitizing-css

You have to install the additional package bleach[css] and pass a CSSSanitizer to the sanitizer. I can do the work, but there's a bunch of other changes to mkdocs that we need to verify doesn't break the docs. On top of that, bleach is deprecated (still going to get security patches I think).

Not sure what the best option is here. However if we keep the current fix, we need to remember that when we upgrade python-markdown to 3.4.x, we need to pass the use_align_attribute=True to TableExtension - otherwise tables will break because they made the new way of doing it the default.

Simple stuff is never simple...

[jeremystretch]

In that case, I have no problem merging this as-is. Thanks for digging so far into this!