MNT: Make a note that setuptools-scm can be unpinned - #31677

…677-on-v3.11.x

Backport PR #31677 on branch v3.11.x (MNT: Make a note that setuptools-scm can be unpinned)

gh-43125: Update Matplotlib to 3.10.9 for pybind11 3.1 compatibility
    
Building Matplotlib 3.10.8 with pybind11 3.1 makes integer arguments to
Agg and FreeType functions emit deprecation warnings intended for
floats. Ordinary plot rendering then produces unexpected output and
fails Sage doctests, as seen in the classic macOS job of #43112.

Update the Matplotlib SPKG to 3.10.9, which includes the released fix in
matplotlib/matplotlib#31511. This updates the
version and source checksums within the existing 3.10 series.

Validation on macOS arm64 with CPython 3.15:

- Built both 3.10.8 and 3.10.9 from their official source archives with
pybind11 3.1.0 and Sage's system FreeType/Qhull options.
- Reproduced warnings for integer Agg coordinates and FreeType image
dimensions on 3.10.8; both are warning-free on 3.10.9. Actual float
arguments still warn.
- Rendered the same PNG with both builds: 38 spurious deprecation
warnings on 3.10.8, zero on 3.10.9.
- All 1,074 Sage doctests in `graphics.py`, `plot.py`, `text.py`, and
`multigraphics.py` pass with the locally built 3.10.9 wheel.
- Built 3.10.9 with Sage's unchanged `sdh_build_wheel` helper and
current `meson-python` 0.19.0 / `setuptools_scm` 10.0.5, exercising its
existing `--no-isolation --skip-dependency-check` fallback.
- Verified the source archive against PyPI's SHA256 and Sage's package
checksum checker.

The new upstream `setuptools_scm<10` build constraint addresses editable
Matplotlib installs; upstream explicitly permits downstream wheel
builders to relax it in
matplotlib/matplotlib#31677. The normal wheel
build above succeeds with Sage's current toolchain.

### 📝 Checklist

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes. (Existing plotting
doctests and a local upstream regression reproducer were run.)
- [ ] I have updated the documentation and checked the documentation
preview. (Not applicable to this SPKG update.)

### ⌛ Dependencies

None; this targets `develop` independently of #43112.
    
URL: #43125
Reported by: Dima Pasechnik
Reviewer(s):