Add PyUnicodeWriter API

[vstinner]

TODO: update API doc.

[vstinner]

Wait until python/cpython#119184 is merged.

[seberg]

It seems to me that PyUnicodeWriter_WriteRepr and PyUnicodeWriter_WriteStr are missing the static inline to ensure multiple includes don't collide?

[vstinner]

It seems to me that PyUnicodeWriter_WriteRepr and PyUnicodeWriter_WriteStr are missing the static inline to ensure multiple includes don't collide?

Oops, fixed by #104.

[Rogdham]

#ifPY_VERSION_HEX<0x030E0000&&PY_VERSION_HEX >= 0x03060000&& !defined(PYPY_VERSION)
// snipPyErr_Format(PyExc_TypeError, "expect str, not %T", str);

I did not experience the issue myself, but I think the %T format was introduced in gha-116839 (PEP-737), so may not be available on all versions of CPython?

[vstinner]

@Rogdham:

I did not experience the issue myself, but I think the %T format was introduced in python/cpython#116839 (PEP-737), so may not be available on all versions of CPython?

Oops, well spotted: I created #146 to fix the issue.