Ship type stubs and allow installing on Python 3.9

[avgvi]

Two packaging changes that unblock libraries switching to the fork.

Type stubs. The package now ships py.typed and .pyi stubs, so mypy and pyright check code that uses diskcache_community without a separate stubs package. Libraries that type-check strictly against the diskcache-stubs package (pySigma, for example) fail with "missing library stubs or py.typed marker" on 5.8.0.

• They cover the upstream API and the fork's additions: SafeDisk and with_types, Disk.with_options and on_error, configure, default_disk, ReadOnlyError, Cache(read_only=...) and the read_only property, store_time on get, iterkeys(tag=...), disk=None defaults, and disk on Deque, Index, FanoutCache.deque and FanoutCache.index.
• __delitem__ and reset move from the shared protocol to Cache, and FanoutCache declares its own, matching the runtime.
• Cache._con and Cache._sql are declared, since subclasses such as dvc-data's HashesCache run SQL through them, and Index.fromcache returns Index[Any, Any] so callers need no annotation.
• .pylintrc skips .pyi files; blue and isort format them.

Python 3.9. requires-python is now >=3.9. The code already runs there: the full suite passes on CPython 3.9.25 (517 passed, 98.79 % coverage). Until now pip refused to install the fork on 3.9, which blocks projects that still support it, such as dvc-data.

Checked:

• python -m mypy.stubtest diskcache_community reports no differences from the runtime, with Django installed (allowlisted: the stub-only _types module, and DjangoCache in __init__, which exists only when Django is installed).
• pySigma's strict mypy run passes against a wheel from this branch with its imports switched to diskcache_community and diskcache-stubs removed; against 5.8.0 it fails with the missing-stubs error.
• dvc-data's mypy run passes against a wheel from this branch once its own Cache subclass is fixed (a type[...] annotation, store_time on its get override and one type: ignore), and its 219 tests pass on Python 3.12 and 3.9.
• The wheel contains py.typed and all seven .pyi files.
• The full test suite passes on Python 3.12 (517 passed, 98.99 %) and 3.9; bluecheck, isortcheck, flake8, mypy, pylint (on 3.12, as in CI) and doc8 pass.