It didn't like the fact I was trying to save some copy-paste work.

We're now generating a section for this metadata.  While we're here, add
scripts and GUI scripts to that metadata.

(Rebased on 8f007bc)

We're not calling or accessing any instance methods.  While we're here,
fill out the documentation a little more.

This is needed to be able to use a sequence of them to construct a
`dict` using the `dict` constructor, as the latter does not like getting
a sequence of `dict`s as input.

This may generate an empty `dict` if `setuptools` gives us an empty
`list` as input, but 99% of the time we should be fine.  We'll re-visit
this if it becomes a problem later.

Use "doctests" style to document the function's behaviour, with more
formal definitions below.

We don't actually need to expose them on the class itself since they
reference none of the class instance fields.  Probably makes more sense
to make these discrete functions.

Since this isn't a public-facing API, it probably makes more sense to
make this a doc test.  It also helps document it.

The only time this is spelt entry-points, is in the TOML output.
In documentation, it is "entry points" (space).  In code, we'll use
`entry_points` (underscore) -- _unless_ it's the TOML output.