Patch version 3:

* Use also Argument Clinic for Struct.__init__()
* Rename fmt to format: in the code and docstring. By the way, Struct docstring was wrong: Struct() accepts a 'format' keyword argument, not 'fmt'
* Use Py_buffer converter for the buffer parameter, except for iter_unpack()
* Document that unpack_from() now accepts keywords and write an unit test
* Fix Struct.__init__() error message: the method accepts also Unicode
* Document that Struct accepts str encodable to ASCII and bytes

I would be nice to use Py_buffer format for the "buffer" argument of iter_unpack(), but Argument Clinic calls PyBuffer_Release(&buffer), whereas the buffer should stay alive after the function exit. Is there a way to clone/copy a Py_buffer?

For the fmt/format argument: s_init() uses custom code to accept Unicode encodable to ASCII and bytes objects. Using Argument Clinic to check fmt type would require to write a converter. I would prefer to not make too many changes in a single patch, and I don't know how to write such converter. I suggest to do that later and keep the existing code. It seems like all functions getting a format ends in s_init() to check the format argument.