improve coverage for pyarrow.struct typehint

[ben-freist]

Hey,
I ran into a mypy error and would like to fix it upstream.
This is from the docstring of pyarrow.struct.

import pyarrow as pa
from typing import List, Tuple

fields: List[Tuple[str, pa.DataType]] = [
    ("f1", pa.int32()),
    ("f2", pa.string()),
]
struct_type = pa.struct(fields)

To make the last line pass mypy Iterable[Tuple[str, DataType]] needs to be added to the stub.

The actual implementation is even more permissive, is that needed? I.e. Iterable[Tuple[x]] works as long as you can feed x into pyarrow.field.
(https://github.com/apache/arrow/blob/f9a6edac9f175de3ad993887470dd1dff4f151c1/python/pyarrow/types.pxi#L5195)