Summary

Partial answer to #39: this fixes the hints which were wrong and makes
the ones already there useful, but it does not describe every function yet.

• py.typed: the package never shipped the PEP 561 marker, so type
  checkers ignored its hints once installed, whatever the source contained.
  It is now declared in pyproject.toml and manifest.in.
• Wrong hints fixed. Running mypy for the first time reported 3 errors,
  and fixing them showed a few more:
  • IJavaStreamParser._read_content had no body: calling it on the
    interface returned None instead of raising NotImplementedError,
    unlike every other method of that class.
  • _do_classdesc and _read_classdesc were declared as returning a
    JavaClassDesc, but they return None when the stream holds TC_NULL.
    Writing that down showed 4 places using the result without checking
    it, which raised an AttributeError on such a stream: reading an
    object, a class and an array now report a clear ValueError instead.
  • _read_content and the lists of annotations have the same story: None
    is a valid content and is kept as such.
  • DefaultObjectTransformer.create indexed its type map with a name which
    can be None.
• The hints are now checked: [tool.mypy] in pyproject.toml and a
  types job in the CI. mypy is green over the 25 modules.
• Type comments added to javaobj/utils.py, shared by the three
  implementations.

Left for 0.7

About 160 functions still have no hints, mostly in the beans, transformers
and unmarshaller of v1 and v2. That is a mechanical change over a thousand
lines, and each annotation makes mypy check the body of the function,
which surfaces more work in turn: it is better done module by module than in
the release fixing four parser bugs. check_untyped_defs and
disallow_untyped_defs are set to false in [tool.mypy] on purpose:
turning them on is the measure of that progress.

Test plan

• mypy: Success: no issues found in 25 source files.
• Full test suite green: the hints of v1 and v2 are comments, so they change
  nothing at runtime, but the 4 ValueError guards do run.
• uv build, then checked that py.typed is in the wheel and in the sdist.
• flake8 gate clean.

Contributes to #39