New thread doesn't copy context of the parent thread.

The minimal example to reproduce the issue:
```
from threading import Thread
from contextvars import ContextVar

foo: ContextVar[str] = ContextVar("foo")


def temp():
    print(foo.get())


foo.set("bar")

t = Thread(target=temp)
t.start()
t.join()
```

Is it expected behavior?

PEP 567 I didn't find anything regarding this case.

Trust me, it's too late to fix this in 3.10.