make api more python3-dict-like

[ThomasWaldmann]

the current api is not quite like a dictionary, but rather like borgbackup's hashindex api is like.

considering that borgbackup has a wrapper class around borghash classes anyway, we can change borghash api to be more like python3 dict api.

maybe not implement everything, but name the frequently used stuff as usual:

• .iteritems -> .items
• __init__: make all special stuff pure kwargs
• .update
• also implement __iter__, .keys and .values

Check:

• collections.abc.Mapping
• collections.abc.MutableMapping https://github.com/python/cpython/blob/3.13/Lib/_collections_abc.py#L924

[ThomasWaldmann]

Note: subclassing from collections.abc.MutableMapping didn't work.

Cython complained (IIRC) that this is not an extension class.