Fix Ruff warnings

This package has lots of Ruff warnings.  For example [here](https://github.com/bdaiinstitute/spatialmath-python/blob/master/spatialmath/spline.py#L195):

```
if check_type is "local":
```

Use `==` to compare constant literals Ruff [F632](https://docs.astral.sh/ruff/rules/is-literal)