gh-106320: Remove private _PyErr_ChainExceptions() (#108713)

Remove _PyErr_ChainExceptions(), _PyErr_ChainExceptions1() and
_PyErr_SetFromPyStatus() functions from the public C API.

* Move the private _PyErr_ChainExceptions() and
  _PyErr_ChainExceptions1() function to the internal C API
  (pycore_pyerrors.h).
* Move the private _PyErr_SetFromPyStatus() to the internal C API
  (pycore_initconfig.h).
* No longer export the _PyErr_ChainExceptions() function.
* Move run_in_subinterp_with_config() from _testcapi to
  _testinternalcapi.