Updated int type for PEP353

[tai]

Due to breaking change in Python API starting from 3.10, python-nkf does not function anymore.

$ ipython
Python 3.10.7 (tags/v3.10.7:6cc6b13, Sep  5 2022, 14:08:36) [MSC v.1933 64 b
it (AMD64)]
Type 'copyright', 'credits' or 'license' for more information
IPython 8.5.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: import nkf

In [2]: nkf.guess('hogehoge')
---------------------------------------------------------------------------
SystemError                               Traceback (most recent call last)
Cell In [2], line 1
----> 1 nkf.guess('hogehoge')

SystemError: PY_SSIZE_T_CLEAN macro must be defined for '#' formats

This is caused by an intentional incompatibility introduced from 3.10:

• PEP 353: Drop support for PyArg_ParseTuple() "#" formats when PY_SSIZE_T_CLEAN is not defined python/cpython#85115
• https://peps.python.org/pep-0353/

This patch updates int type to Py_ssize_t and adds PY_SSIZE_T_CLEAN, so it will work again.
That said, since int values are still used everywhere in nkf.c, it still needs further update to handle input longer than 32bit limit. This patch does not handle that.