bpo-40521: Per-interpreter interned strings (GH-20085)

Make the Unicode dictionary of interned strings compatible with
subinterpreters.

Remove the INTERN_NAME_STRINGS macro in typeobject.c: names are
always now interned (even if EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
macro is defined).

_PyUnicode_ClearInterned() now uses PyDict_Next() to no longer
allocate memory, to ensure that the interned dictionary is cleared.