gh-102304: Move _Py_RefTotal to _PyRuntimeState (gh-102543)

The essentially eliminates the global variable, with the associated benefits. This is also a precursor to isolating this bit of state to PyInterpreterState.

Folks that currently read _Py_RefTotal directly would have to start using _Py_GetGlobalRefTotal() instead.

https://github.com/python/cpython/issues/102304