gh-111089: PyUnicode_AsUTF8() now raises on embedded NUL (#111091)

* PyUnicode_AsUTF8() now raises an exception if the string contains
  embedded null characters.
* Update related C API tests (test_capi.test_unicode).
* type_new_set_doc() uses PyUnicode_AsUTF8AndSize() to silently
  truncate doc containing null bytes.

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>