Fix AttributeError when one of css_files is a string

[mitya57]

Unfortunately, the last version of #1528 still did not work properly in some cases.

When css_files comes from Sphinx, every item of this list is an instance of _CascadingStyleSheet class, and css_tag works with it.

However, some projects override css_files in their templates:

• https://github.com/laspy/laspy/blob/2.5.1/docs/_templates/layout.html

or pass css_files to HTML context explicitly from conf.py:

• https://review.coreboot.org/plugins/gitiles/coreboot/+/refs/tags/4.20.1/Documentation/conf.py#91
• https://github.com/isl-org/Open3D/blob/v0.17.0/docs/conf.in.py#L160

To make build of these projects succeed, we need to support css_file being a string.
Here I copied the logic from Sphinx itself.

See this Debian bug for the logs of the failed builds.

[agjohnson]

Bah, I thought we were about ready for a 2.0 release too. Thanks for the heads up and the keen eye on this @mitya57

The changes here look good to me. I don't have any strong input on the most correct logic to use here either though. But we are back to close to what we had originally, so I would assume we are in a safer place.

I was going to put out 2.0 tomorrow, but I'll aim for a 2.0rc4 today instead.

[agjohnson]

2.0.0rc4 is now out!