[logic][itertoolz] merge_sorted

[hunterhogan]

The Python and Cython implementations handle keyword arguments differently.

Observed implementation difference:

• humpy_toolz/itertoolz.py:773-795 has an explicit Python signature with only key as a supported keyword argument.
• humpy_cytoolz/itertoolz.pyx:313-331 accepts **kwargs, checks only whether 'key' in kwargs, and otherwise ignores the contents of kwargs.

Observable difference:

• Python rejects unexpected keyword arguments before executing merge_sorted.
• Cython accepts unexpected keyword arguments and ignores them when they are not named key.

This issue only records the difference between implementations.