Fix FanoutCache.__getattr__ breaking hasattr() with AssertionError

[cristianchiriac]

Fixes #353.

Bug

FanoutCache.__getattr__ uses a bare assert to reject unknown attribute names:

def __getattr__(self, name):
    safe_names = {'timeout', 'disk'}
    valid_name = name in DEFAULT_SETTINGS or name in safe_names
    assert valid_name, 'cannot access {} in cache shard'.format(name)
    return getattr(self._shards[0], name)

hasattr() only swallows AttributeError; any other exception propagates. So hasattr(FanoutCache(), 'anything_unknown') raises AssertionError instead of returning False. This breaks code that probes objects with hasattr(), such as pympler's asizeof, which crashed the pympler debug toolbar panel when it checked hasattr(cache, '__slots__').

Fix

Raise AttributeError instead of asserting, which is what __getattr__ is expected to raise for an unknown attribute per the data model, and is what hasattr() actually checks for.

Test plan

• Added test_getattr_invalid_name_raises_attribute_error to tests/test_fanout.py, asserting both that accessing an invalid name raises AttributeError and that hasattr() returns False rather than raising.
• Verified the new test fails against the unmodified code (AssertionError instead of AttributeError) and passes with the fix.
• pytest tests/test_fanout.py: 43 passed.
• pytest tests/ (excluding the Django-backend tests, since django isn't installed in this environment): 137 passed.