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.

Python/pylifecycle.c

@@ -1573,6 +1573,8 @@ finalize_interp_types(PyThreadState *tstate)
     _PyFrame_Fini(tstate);
     _PyAsyncGen_Fini(tstate);
     _PyContext_Fini(tstate);
+    // Call _PyUnicode_ClearInterned() before _PyDict_Fini() since it uses
+    // a dict internally.
     _PyUnicode_ClearInterned(tstate);
 
     _PyDict_Fini(tstate);