Sphinx 3 issues

[jamesjer]

The Fedora Linux distribution has started the process of switching from sphinx 2 to sphinx 3. The yices package fails to build its documentation with sphinx 3. There are two issues:

1. sphinx.locale.l_, which was deprecated in sphinx 2, has been removed entirely from sphinx 3. This affects both sphinx extensions: cenum.py and cheader.py. The _ function should be used instead.
2. Sphinx 3 provides an enum role and an enum directive, so cenum.py is not needed. In fact, it doesn't work at all due to changes in CDomain in sphinx 3.

[BrunoDutertre]

Thanks Jerry. I'll look into this. Paradoxically, Sphinx's own documentation is still as horrible
as I remember.

[BrunoDutertre]

I've tried to fix conf.py in to handle this in 52d025a.
@jamesjer Can you check whether it works for you?

[jamesjer]

Thanks for the quick fix! That commit does indeed resolve the issue.