Fix missing colons in dask/dask

[gforsyth]

This fix updates the layout.html with changes I grabbed from
readthedocs/sphinx_rtd_theme#838 that relate to the version of the
htmlwriter that is used.

This should resolve dask/dask#6696 once a new release of the theme is
cut (I've bumped the version in __init__.py) and the update is pulled
in there.

[gforsyth]

@jacobtomlinson -- I saw your note about readthedocs/sphinx_rtd_theme#853 and so since it has been merged I've switched this back to using extends

I think the only extra bit since then is the SOCIAL GRAPH stuff, but if you have a quick second to glance over this and make sure I didn't miss something obvious, it would be greatly appreciated.

FWIW, I've built the docs locally with these changes and everything looks good.

[mrocklin]

I think that @jacobtomlinson is out for the next few weeks on parental leave. I don't think that we should wait for him as I believe that he may have turned off notifications.

Should we merge this in?

[mrocklin]

Also, I think that when we merge things are set up to automatically release

[gforsyth]

It builds fine on my end, so I think we're good to merge. (famous last words)

[jrbourbeau]

Thanks @gforsyth!

For reference, we have a GitHub actions workflow set up to automatically push to PyPI on tagged commits. I added the 1.3.3 tag over in 084b696

[gforsyth]

Awesome, thanks @jrbourbeau !