bpo-39984: Move pending calls to PyInterpreterState (GH-19066)

If Py_AddPendingCall() is called in a subinterpreter, the function is
now scheduled to be called from the subinterpreter, rather than being
called from the main interpreter.

Each subinterpreter now has its own list of scheduled calls.

* Move pending and eval_breaker fields from _PyRuntimeState.ceval
  to PyInterpreterState.ceval.
* new_interpreter() now calls _PyEval_InitThreads() to create
  pending calls lock.
* Fix Py_AddPendingCall() for subinterpreters. It now calls
  _PyThreadState_GET() which works in a subinterpreter if the
  caller holds the GIL, and only falls back on
  PyGILState_GetThisThreadState() if _PyThreadState_GET()
  returns NULL.
diff --git a/Modules/signalmodule.c b/Modules/signalmodule.c
index 1028531..a26ae7d 100644
--- a/Modules/signalmodule.c
+++ b/Modules/signalmodule.c
@@ -304,7 +304,7 @@
                 if (wakeup.warn_on_full_buffer ||
                     last_error != WSAEWOULDBLOCK)
                 {
-                    /* Py_AddPendingCall() isn't signal-safe, but we
+                    /* _PyEval_AddPendingCall() isn't signal-safe, but we
                        still use it for this exceptional case. */
                     _PyEval_AddPendingCall(tstate,
                                            report_wakeup_send_error,
@@ -323,7 +323,7 @@
                 if (wakeup.warn_on_full_buffer ||
                     (errno != EWOULDBLOCK && errno != EAGAIN))
                 {
-                    /* Py_AddPendingCall() isn't signal-safe, but we
+                    /* _PyEval_AddPendingCall() isn't signal-safe, but we
                        still use it for this exceptional case. */
                     _PyEval_AddPendingCall(tstate,
                                            report_wakeup_write_error,