Tighten client closed-state behaviour

[lovelydinosaur]

Closes #1317
Closes #1332

Tighens up the client open/closed state by making it tri-state under the hood: UNOPENED/OPENED/CLOSED.

End result of the changes here, are that only the following styles are valid...

client=httpx.Client()
client.request(...)
client.close() # If omitted, this will be called on `__del__` with sync client, or raise a warning on `__del__` with async client.# Cannot send requests anymore.

Or...

withhttpx.Client() asclient:
client.request(...)
# Cannot send requests anymore.

Because we're tri-state now we can make sure that simply importing but not using a client won't raise errors.
(Because under the hood, the client is initially in an UNOPENED state.)

# If we don't actually use this client, then `__del__` won't raise a warning here.client=httpx.AsyncClient()

• Clients may no longer used once closed. Calling client.get(...) and pals will raise a runtime error if attempted.
• Clients that have not sent a request should still call .close() on the underlying transports if closed.
• AsyncClients that have not been opened should not raise a warning if deleted.

[florimondmanca]

👍👍👍

Fab!