sagemathgh-37287: sagemath-standard: include sage_setup in sdist
    
This makes it so that `sage_setup` is included in `sagemath_standard`
for convenience and ease of installation.

Having `sage_setup` as a separate distribution leads to chicken-and-egg
problem. For example, this is what happens when I try to build
`sagemath-standard` from sagemath#37270:
```
cd pkgs/sagemath-standard
$ python -m build -s
* Creating venv isolated environment...
* Installing packages in isolated environment... (cypari2 >=2.1.1,
cysignals >=1.10.2, cython >=3.0, != 3.0.3, <4.0, gmpy2 ~=2.1.b999,
jinja2 >=3.0, jupyter_core >=4.6.3, memory_allocator, numpy >=1.19,
pkgconfig, pplpy >=0.8.6, sage-conf ~= 10.3b7, sage-setup ~= 10.3b7,
sage_setup[autogen], setuptools >= 68.1.1, wheel >=0.36.2)

[... snip ...]

ERROR: Could not find a version that satisfies the requirement sage-
setup~=10.3b7 (from versions: 9.4rc1)
ERROR: No matching distribution found for sage-setup~=10.3b7

[...]
```
This is because the version of sage-setup which is available on pypi
requires python <= 3.11. Even if the branch I'm trying to build allows
python 3.12, I cannot use it.

The main idea of this change is that `sagemath-standard` sdists are
self-contained and include the exact matching version of sage-setup. The
self-contained part is good for the pypi sdist, but the matching version
allows for greater flexibility when testing, given that the
implementations of `sagemath-standard/setup.py` and `sage_setup` are so
intrincately connected and really need to match.

### 📝 Checklist

- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
    
URL: sagemath#37287
Reported by: Gonzalo Tornaría
Reviewer(s): Dima Pasechnik, Tobias Diez