[python-package] ignore mypy errors related to ctypes string buffers

[jameslamb]

Contributes to #3867.

A few places in the Python package pre-allocate C char arrays with ctypes.create_string_buffer(), then pass a C array of pointers to those buffers through LightGBM's C API.

mypy is not happy about the way that's done:

python-package/lightgbm/basic.py:2951: error: Array constructor argument 1 of type "map[int]" is not convertible to the array element type "Iterable[c_char_p]"  [misc]
python-package/lightgbm/basic.py:2965: error: Array constructor argument 1 of type "map[int]" is not convertible to the array element type "Iterable[c_char_p]"  [misc]
python-package/lightgbm/basic.py:4631: error: Array constructor argument 1 of type "map[int]" is not convertible to the array element type "Iterable[c_char_p]"  [misc]
python-package/lightgbm/basic.py:4645: error: Array constructor argument 1 of type "map[int]" is not convertible to the array element type "Iterable[c_char_p]"  [misc]
python-package/lightgbm/basic.py:4855: error: Array constructor argument 1 of type "map[int]" is not convertible to the array element type "Iterable[c_char_p]"  [misc]
python-package/lightgbm/basic.py:4871: error: Array constructor argument 1 of type "map[int]" is not convertible to the array element type "Iterable[c_char_p]"  [misc]

The way the package does this appears to follow all of the guidance from ctypes in https://docs.python.org/3/library/ctypes.html#arrays, so I suspect this might be an issue with the ctypes type hints or maybe mypy's type inference. I reported that at python/mypy#6212 (comment).

This PR proposes just ignoring those issues with #type: ignore comments.