bpo-39984: trip_signal() uses PyGILState_GetThisThreadState() (GH-19061)

bpo-37127, bpo-39984:

* trip_signal() and Py_AddPendingCall() now get the current Python
  thread state using PyGILState_GetThisThreadState() rather than
  _PyRuntimeState_GetThreadState() to be able to get it even if the
  GIL is released.
* _PyEval_SignalReceived() now expects tstate rather than ceval.
* Remove ceval parameter of _PyEval_AddPendingCall(): ceval is now
  get from tstate parameter.