Require typing_extensions with Self support on Python 3.10 (#233)

## Summary

On Python 3.10, `spatialmath.base._types_39` imports `Self` from
`typing_extensions`. The current unbounded dependency allows
`typing_extensions==3.10.0.2`, which lacks `Self` and makes `import
spatialmath` fail. Require `typing_extensions>=4.0.0`, the first release
with `Self`, so the package metadata reflects this import requirement.

## Validation

- With `typing_extensions==3.10.0.2`, reproduced `ImportError: cannot
import name 'Self' from 'typing_extensions'` before the metadata change.
- Reinstalled the editable package after the change; dependency
resolution upgraded `typing_extensions` to 4.16.0 and `import
spatialmath` succeeded.
- Also checked that `import spatialmath` succeeds with the minimum
allowed version, 4.0.0.
- With the resolved current dependency set, `pytest -q
--disable-warnings`: **347 passed, 3 skipped** (Python 3.10.12); `git
diff --check` passed.

Fixes #78. Prepared and validated with OpenAI Codex assistance.

Signed-off-by: 李永祺 <doribelove@gmail.com>

Author: Doribelove

pyproject.toml

@@ -37,7 +37,7 @@ dependencies = [
     "scipy",
     "matplotlib",
     "ansitable",
-    "typing_extensions",
+    "typing_extensions>=4.0.0",
     "pre-commit",
 ]