Prepare package for PyPI: fix versioning, lazy imports, Python constraint

[Copilot]

Package wasn't publishable to PyPI: version was hardcoded in two places, import pyfuse eagerly pulled in optional backend deps (redis, aio-pika), and requires-python >=3.13 was unnecessarily restrictive given no 3.13-specific features are used.

Versioning — single source of truth

• pyproject.toml is authoritative; pyfuse/core/version.py reads it at runtime via importlib.metadata, falls back to _FALLBACK_VERSION for editable/source installs
• Exposes pyfuse.__version__ in __init__.py and __all__

Lazy backend imports

• RedisBackend import in remote.py moved inside _create_backend(), matching existing pattern for LocalBackend/RabbitMQBackend
• backends/__init__.py only re-exports the abstract Backend base

# Before: top-level import — fails without redis installed
from pyfuse.worker.backends.redis import RedisBackend

# After: lazy, inside _create_backend()
if scheme in ("redis", "rediss"):
    from pyfuse.worker.backends.redis import RedisBackend
    return RedisBackend(url, **kwargs)

Python version constraint

• Lowered requires-python from >=3.13 to >=3.10 (sys.stdlib_module_names is the floor)
• Updated classifiers and README badge

Isolated installation tests (tests/test_packaging.py)

• Version consistency: fallback matches pyproject.toml, __version__ is exported
• Installs in a fresh venv without extras, verifies import + core API + CLI entrypoint