refactor(base): collapse the three-way Python-version type shim into one file (#212)

## Summary

Step 1 of a broader type-hint cleanup pass (cc @jbarry-bdai — following
on from our email thread on this). Opening this narrow, mechanical piece
first to get it moving while the rest gets scoped out.

`spatialmath/base/types.py` dispatched on `sys.version_info` across
three duplicate files (`_types_35.py`, `_types_39.py`, `_types_311.py`,
~465 lines total), from when Python 3.5-3.10 support coexisted.
`pyproject.toml` has required `>=3.10` since #186, so `_types_35.py`
could never actually be reached, and `_types_39`/`_types_311` differed
only in where `Self` is imported from (`typing_extensions` vs `typing`).
This collapses all three into a single ~170-line file with a two-line
version check. No aliases or exported names changed — purely a dedup,
verified by a full test suite pass (343 passed).

## What's next

This is deliberately the smallest, lowest-risk slice. A quick survey of
the rest of the package turned up the following, to be tackled in
follow-up PRs:

- **Annotation coverage is uneven.** 838 functions/methods surveyed
across `spatialmath/` and `spatialmath/base/`: 57.5% fully annotated
overall. Most of `base/` is in decent shape (72-96%), but `animate.py`
is only 6% annotated. Top-level is weaker — `pose2d.py`,
`spatialvector.py`, and `twist.py` are all at 0%.
- **Old-style typing syntax is still widespread.** `Optional[X]`,
`Union[X, Y]`, `List[X]` etc. appear ~400+ times across the package
(heaviest in `graphics.py` at 124 occurrences) and can move to `X |
None` / `X | Y` / `list[X]` now that 3.10+ is the floor.

Planned sequencing: this shim collapse, then a mechanical
syntax-modernization pass, then filling the real coverage gaps
(`twist.py`/`spatialvector.py`/`pose2d.py` first, then `animate.py`) as
their own PR(s) since those need actual thought rather than
find-replace.

Author: petercorke