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

spatialmath/base/types.py dispatched on sys.version_info across
_types_35.py, _types_39.py and _types_311.py, from when Python 3.5-3.10
support coexisted. pyproject.toml has required Python >=3.10 since #186,
so _types_35.py could never be reached, and _types_39/_types_311 differed
only in where Self is imported from. Collapse to a single file with a
two-line version check; no change to the names or aliases it exports.

Author: petercorke