Habachen

Minor updates

- The *\_\_text_signature__* attribute (used by the inspect module, for example) for the habachen functions is now available.

- Improved portability.

- Added pyproject.toml, following PEP 517/518.

- Slight performance improvement: now use the stack memory for fixed-size temporary buffers in *hiragana_to_katakana()* and *katakana_to_hiragana()*.

- More consistent and precise error handling: assure that *hiragana_to_katakana()* raises a ValueError when *hankaku* option is set to True and the *ignore* optional string contains characters that are not convertible to hankaku katakana. Previously, even though neither 'ゐ' (U+3090) nor 'ゟ' (U+309F) has a corresponding hankaku katakana, only the latter triggered an error.

- Added wheels for major platforms on PyPI, by using cibuildwheel.

- Speed up katakana_to_hiragana() function with gcc on x86-64.

First public release