bpo-42296: On Windows, fix CTRL+C regression (GH-23257)

On Windows, fix a regression in signal handling which prevented to
interrupt a program using CTRL+C. The signal handler can be run in a
thread different than the Python thread, in which case the test
deciding if the thread can handle signals is wrong.

On Windows, _PyEval_SignalReceived() now always sets eval_breaker to
1 since it cannot test _Py_ThreadCanHandleSignals(), and
  eval_frame_handle_pending() always calls
  _Py_ThreadCanHandleSignals() to recompute eval_breaker.