gh-133968: Add PyUnicodeWriter_WriteASCII() function (#133973)

Replace most PyUnicodeWriter_WriteUTF8() calls with
PyUnicodeWriter_WriteASCII().

Unrelated change to please the linter: remove an unused
import in test_ctypes.

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>