bpo-40521: Make Unicode latin1 singletons per interpreter (GH-21101)

Each interpreter now has its own Unicode latin1 singletons.

Remove "ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS"
and "ifdef LATIN1_SINGLETONS": always enable latin1 singletons.

Optimize unicode_result_ready(): only attempt to get a latin1
singleton for PyUnicode_1BYTE_KIND.