Bugs: packaging, CI, docs and test suite (0.28.1) #32

Defects found while reviewing 0.28.1 (each one reproduced). They are fixed as part of the 1.0.0 redesign on the v1-redesign branch; every item is ticked when the commit that fixes it lands.

• C08/C44 setup.cfg puts python_requires and include_package_data under [options.packages.find], so the built wheel has no Requires-Python. (8f78371)
• C45 Wheel contents depend on setuptools ≥ 61 auto-discovery (subpackages have no __init__.py, unpinned build requirement); older setuptools builds an empty wheel. (8f78371)
• C50 Makefile uses .PHONY = target (a variable assignment) instead of .PHONY: target; make test silently does nothing if a file named test exists. (4fa9bac)
• C43 make test imports tests/numbers as the top-level package numbers, shadowing the stdlib module (breaks as soon as decimal/fractions is imported). (4fa9bac) (tests/numbers still shadows numbers when tests/ itself is on sys.path, until B3.5 deletes it; AGENTS.md says to run tests from the root, 521522a.)
• C55 AGENTS.md claims default pytest discovery does not find the tests; it finds all of them. (f71d20c)
• C52 README: CI badge points to a non-existent test.yml; TOC entry "Nullable Positive Int" links to #positive-int; Non Empty String sections missing from the TOC. (Badge fixed in a0c8e52; the two TOC items are still open.)
• C18 README TOC links "Nullable Positive Int" to the Positive Int section.
• C32/C51 README says NullableUuid4(None).value() returns the string 'None'.
• C47 README IsoDate example ('2023-08-15T04:55:12.076Z') raises on Python 3.9/3.10, which CI claims to support.
• C49 14 security VOs, the abstract base, the nullable factory and the public import API have no tests at all.