Use PyBytesWriter in Unicode codecs

[vstinner]

Replace PyBytes_FromStringAndSize() and _PyBytes_Resize() with PyBytesWriter API (PEP 782) in Unicode codecs

Linked PRs

• gh-139156: Use PyBytesWriter in UTF-32 encoder #139157
• gh-139156: Use PyBytesWriter in UTF-16 encoder #139233
• gh-139156: Use PyBytesWriter in the UTF-7 encoder #139248
• gh-139156: Use PyBytesWriter in PyUnicode_AsUnicodeEscapeString() #139249
• gh-139156: Use PyBytesWriter in PyUnicode_AsRawUnicodeEscapeString() #139250
• gh-139156: Use PyBytesWriter in _PyUnicode_EncodeCharmap() #139251
• gh-139156: Optimize the UTF-7 encoder #139253
• gh-139156: Use PyBytesWriter in PyUnicode_EncodeCodePage() #139259