build: drop oldest-supported-numpy from build requirements (#286)

`pyproject.toml` lists `oldest-supported-numpy` as a build requirement.
spatialmath is pure Python (no extension modules, no `setup.py`), so
numpy is not needed at build time. numpy has also deprecated
`oldest-supported-numpy`.

Build tools that only install the build backend fail on it. One example
is rules_py's uv sdist builder, which fails with `Unmet dependencies:
oldest-supported-numpy` when building spatialmath from a source archive.
This PR leaves `requires = ["setuptools"]`.

### Testing

- `pyproject-build` (isolated) builds the sdist and the wheel.
- The wheel's file list and `METADATA` are identical to a wheel built
from `master`.

Author: taughz

pyproject.toml

@@ -78,7 +78,7 @@ ros-humble = [
 
 [build-system]
 
-requires = ["setuptools", "oldest-supported-numpy"]
+requires = ["setuptools"]
 build-backend = "setuptools.build_meta"
 
 [tool.setuptools]