Replace Py_UNICODE with Py_UCS4

[tpdn]

fix #24

Currently mojimoji cannot be installed on Python 3.12.
This diff avoids this problem by replacing Py_UNICODE with Py_UCS4.

https://docs.python.org/3.12/c-api/unicode.html

Pytest results with Python 3.12 on my MacBook

============================================= 15 passed in 0.01s =============================================
(hoge-3.12) [15:36:38] tpdn@GrandMacdoBookProXP /Users/tpdn/gitlab/tpdn/hoge  
> pytest mojimoji/test_mojimoji.py
============================================ test session starts =============================================
platform darwin -- Python 3.12.0, pytest-7.4.2, pluggy-1.3.0
rootdir: /Users/tpdn/gitlab/tpdn/hoge/mojimoji
collected 15 items                                                                                           

mojimoji/test_mojimoji.py ...............                                                              [100%]

[ikuyamada]

Thank you very much for submitting the pull request!
I fixed mojimoji.pyx based on the suggested changes in this PR.