[3.14] gh-133968: Add fast path to PyUnicodeWriter_WriteStr() (GH-133969) (#133971)

gh-133968: Add fast path to PyUnicodeWriter_WriteStr() (GH-133969)

Don't call PyObject_Str() if the input type is str.
(cherry picked from commit fe9f6e829a535747b1e06d9bfda033a9a47165ed)

Co-authored-by: Victor Stinner <vstinner@python.org>