include slim branch on pyslim build

[petrelharp]

To get latest/ to build I'll need this for now (and I think it's a good time to turn this on).

[petrelharp]

Ah right, this will have to wait for tskit-dev/msprime#2539

[petrelharp]

There's a chicken-and-egg problem here: the subprojects pull their intersphinx mappings from here (for instance, in docs/_config.yml:

        msprime: ["https://tskit.dev/msprime/docs/stable", null]

and so the subproject docs won't build without the main site having built, but the main site won't build without the subprojects. The problem here was that I merged things into pyslim's main before the required changes in SLiM. So, I'm going to have to break some things temporarily to get out of this.

[petrelharp]

Okay - this removes pyslim; I'm going to merge this, then follow up with another PR that puts pyslim back (hopefully).