Remove or clarify `__setitem__` return

[knutnergaard]

The return of the set glyph in BaseLayer.__setitem__ is unusual, and seems to be ignored by Python so that the example code

>>> glyph = layer["A"] = otherGlyph

will return otherGlyph rather than the intended layer["A"].

Is there another reason for the return that should be documented, or should it be removed?