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

[petercorke]

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.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 97.95918% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
spatialmath/base/types.py	97.95%	1 Missing ⚠️

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