Index equality raises or reports equality when comparing non-mappings #387

On master ebfa37cd99d7ef716ec452ad8af4b4276a8e2233, Index comparisons with non-mappings can raise or return a false positive.

from tempfile import TemporaryDirectory
from diskcache import Index

with TemporaryDirectory() as directory:
    index = Index(directory)
    try:
        print(index == [])    # True; expected False
        print(index == None)  # TypeError; expected False
    finally:
        index.cache.close()

A populated Index compared with a same-length list raises AttributeError because lists have no get(). Both equality methods should return NotImplemented for non-mappings so Python can try reflected comparisons, as MutableMapping does. Existing order-sensitive Index/OrderedDict comparisons and order-insensitive ordinary mapping comparisons should remain unchanged.

I reproduced 17 regression failures and prepared a focused fix with mapping compatibility controls. This is separate from #378, which handles missing-key sentinel collisions between mappings. Found through code inspection with OpenAI Codex assistance; no production incident is claimed.