The Python 2 version was slightly outdated compared to the Python 3
version.
The Python 2 version of Headers.__delitem__() had an optional argument
_index_operation that also exists in the implementation. But considering
this is internal use only, it was missing from the Python 3 version, and
it created problems with derived classes, I decided not to add it to
the merged stub. See also pallets/werkzeug#1051.
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