threading.Thread: Add start=False parameter to start immediately the thread

It's common to create multiple threads, start them, and then join them. Example:

threads = [
    threading.Thread(target=run_a),
    threading.Thread(target=run_b),
]
for thread in threads:
    thread.start()
...
for thread in threads:
    thread.join()

I propose adding a start=False parameter to threading.Thread to start immediately the thread. The example would become:

threads = [
    threading.Thread(target=run_a, start=True),
    threading.Thread(target=run_b, start=True),
]
...
for thread in threads:
    thread.join()

The new parameter would solve the old issue gh-93293 which requests to return self in start(). The issue example:

threads = []
for i in range(10):
  thread = threading.Thread(target=fn, args=(i,))
  thread.start()
  threads.append(thread)

for t in threads:
  t.join()

would become:

threads = [threading.Thread(target=fn, args=(i,), start=True) for i in range(10)]

for t in threads:
  t.join()

Linked PRs

• gh-155335

首先检查 threading.Thread 构造函数以及 start() 和 join() 的行为，然后检查 gh-155335 中关联的工作。确定在构造期间启动的 API 语义和兼容性影响；当提议的用法能够正常工作且不改变现有 Thread 行为时，即视为完成。