bpo-40268: Remove explicit pythread.h includes (#19529)

Remove explicit pythread.h includes: it is always included
by Python.h.

Python/ceval.c

@@ -240,7 +240,6 @@ UNSIGNAL_ASYNC_EXC(PyInterpreterState *interp)
 #ifdef HAVE_ERRNO_H
 #include <errno.h>
 #endif
-#include "pythread.h"
 #include "ceval_gil.h"
 
 static void

Python/import.c

@@ -150,8 +150,6 @@ _PyImportZip_Init(PyThreadState *tstate)
    in different threads to return with a partially loaded module.
    These calls are serialized by the global interpreter lock. */
 
-#include "pythread.h"
-
 static PyThread_type_lock import_lock = 0;
 static unsigned long import_lock_thread = PYTHREAD_INVALID_THREAD_ID;
 static int import_lock_level = 0;

Python/pylifecycle.c

@@ -2350,8 +2350,6 @@ Py_ExitStatusException(PyStatus status)
 
 /* Clean up and exit */
 
-#  include "pythread.h"
-
 /* For the atexit module. */
 void _Py_PyAtExit(void (*func)(PyObject *), PyObject *module)
 {

Python/sysmodule.c

@@ -26,7 +26,6 @@ Data members:
 #include "pycore_pymem.h"
 #include "pycore_pystate.h"      // _PyThreadState_GET()
 #include "pycore_tupleobject.h"
-#include "pythread.h"
 #include "pydtrace.h"
 
 #include "osdefs.h"

Python/thread.c

@@ -23,8 +23,6 @@
 
 #include <stdlib.h>
 
-#include "pythread.h"
-
 #ifndef _POSIX_THREADS
 
 /* Check if we're running on HP-UX and _SC_THREADS is defined. If so, then