fix(python): ship a PEP 561 py.typed marker and make the stubs mypy-clean - #243

Problem

The wheels ship .pyi stubs for the whole package, but no py.typed marker (PEP 561), so mypy ignores them and every ioh import degrades to Any (Skipping analyzing "ioh": module is installed, but missing library stubs or py.typed marker). Fixes #237.

Root cause

An empty ioh/py.typed is missing from the package. With the marker in place, mypy starts honouring the stubs — and then reports 22 errors in the stubs themselves, so the marker alone would trade one problem for another.

Fix

• Add the empty ioh/py.typed; wheel.packages = ["ioh"] and the sdist's /ioh/ include pick it up with no packaging changes.
• Fix the 22 stub errors the marker exposes:
  • wrap_problem, Experiment.__init__ and add_custom_problem: None defaults on parameters typed str / Callable / list → explicit | None;
  • Experiment.apply / add_custom_problem: any / callable used as types → typing.Any / typing.Callable[..., typing.Any];
  • iohcpp/logger/__init__.pyi: the property submodule was re-exported under the name property, shadowing the builtin decorator for every class-level @property in the file (9 × "Module not callable"); it is now imported by full path under a local alias;
  • iohcpp/problem/__init__.pyi: the 1-D and 2-D __call__ overloads were indistinguishable to mypy (the Annotated shape metadata is invisible to it), so the 2-D signature was reported as unreachable; the shape is now expressed in the ndarray type parameter;
  • MetaData.__eq__ widened to the object supertype signature, and the unhashable marker kept via an explicit ignore (mypy 2.x no longer accepts a bare __hash__ = None).

Testing

• mypy -p ioh: 22 errors → Success: no issues found in 12 source files.
• Simulated the installed-package experience with a stub-only copy (.pyi + py.typed, no compiled module): reveal_type(ioh.get_problem) now reports the full signature from the issue's expected output instead of Any.