…reter interned_dict

bpo-46430 caused an interesting side effect; the code
`x = 'a'; x[0] is x` no longer returned True. This in turn
is because there are two different cached versions of 'a':

- One that was cached when code in frozen modules was compiled
  (and is stored in the interned_dict)
- One that is stored as a runtime-global object that is used
  during function calls (and is stored in _Py_SINGLETON(strings))

However, some characters do not have this behaviour (for example,
'g', 'u', and 'z'). I suspect it because these characters are not
used in co_consts of frozen modules.

The interned_dict is per interpreter, and is initialized by
`init_interned_dict(PyInterpreterState *)`. The prior implementation
initialize it to an empty dict, which allows code in frozen modules
to use their (different and per interpreter) singleton strings
instead of the runtime-global one.

The new implementation add all runtime-global singleton strings
to the interned_dict when it initialized, causing the frozen
modules to use the same immortal singleton string and for
`x = 'a'; x[0] is x` to return True.

Using it in the intern dict seems to causes decrementation
of its reference count elsewhere that cannot be solved
with Py_INCREF. However, updating the tuple inside
intern_string_constants does not cause the extra
decrementation.